C#: Restructure code prior move to .NET Core

The main focus here was to remove the majority of code that relied on
Mono's embedding APIs, specially the reflection APIs. The embedding
APIs we still use are the bare minimum we need for things to work.
A lot of code was moved to C#. We no longer deal with any managed
objects (`MonoObject*`, and such) in native code, and all marshaling
is done in C#.

The reason for restructuring the code and move away from embedding APIs
is that once we move to .NET Core, we will be limited by the much more
minimal .NET hosting.

PERFORMANCE REGRESSIONS
-----------------------

Some parts of the code were written with little to no concern about
performance. This includes code that calls into script methods and
accesses script fields, properties and events.
The reason for this is that all of that will be moved to source
generators, so any work prior to that would be a waste of time.

DISABLED FEATURES
-----------------

Some code was removed as it no longer makes sense (or won't make sense
in the future).
Other parts were commented out with `#if 0`s and TODO warnings because
it doesn't make much sense to work on them yet as those parts will
change heavily when we switch to .NET Core but also when we start
introducing source generators.
As such, the following features were disabled temporarily:
- Assembly-reloading (will be done with ALCs in .NET Core).
- Properties/fields exports and script method listing (will be
  handled by source generators in the future).
- Exception logging in the editor and stack info for errors.
- Exporting games.
- Building of C# projects. We no longer copy the Godot API assemblies
  to the project directory, so MSBuild won't be able to find them. The
  idea is to turn them into NuGet packages in the future, which could
  also be obtained from local NuGet sources during development.

modules/mono/signal_awaiter_utils.cpp

@@ -32,16 +32,15 @@
 
 #include "csharp_script.h"
 #include "mono_gd/gd_mono_cache.h"
-#include "mono_gd/gd_mono_class.h"
-#include "mono_gd/gd_mono_marshal.h"
 #include "mono_gd/gd_mono_utils.h"
 
-Error gd_mono_connect_signal_awaiter(Object *p_source, const StringName &p_signal, Object *p_target, MonoObject *p_awaiter) {
+Error gd_mono_connect_signal_awaiter(Object *p_source, const StringName &p_signal, Object *p_target, GCHandleIntPtr p_awaiter_handle_ptr) {
 	ERR_FAIL_NULL_V(p_source, ERR_INVALID_DATA);
 	ERR_FAIL_NULL_V(p_target, ERR_INVALID_DATA);
 
 	// TODO: Use pooling for ManagedCallable instances.
-	SignalAwaiterCallable *awaiter_callable = memnew(SignalAwaiterCallable(p_target, p_awaiter, p_signal));
+	MonoGCHandleData awaiter_handle(p_awaiter_handle_ptr, gdmono::GCHandleType::STRONG_HANDLE);
+	SignalAwaiterCallable *awaiter_callable = memnew(SignalAwaiterCallable(p_target, awaiter_handle, p_signal));
 	Callable callable = Callable(awaiter_callable);
 
 	return p_source->connect(p_signal, callable, Object::CONNECT_ONESHOT);
@@ -51,7 +50,7 @@ bool SignalAwaiterCallable::compare_equal(const CallableCustom *p_a, const Calla
 	// Only called if both instances are of type SignalAwaiterCallable. Static cast is safe.
 	const SignalAwaiterCallable *a = static_cast<const SignalAwaiterCallable *>(p_a);
 	const SignalAwaiterCallable *b = static_cast<const SignalAwaiterCallable *>(p_b);
-	return a->awaiter_handle.handle == b->awaiter_handle.handle;
+	return a->awaiter_handle.handle.value == b->awaiter_handle.handle.value;
 }
 
 bool SignalAwaiterCallable::compare_less(const CallableCustom *p_a, const CallableCustom *p_b) {
@@ -105,38 +104,26 @@ void SignalAwaiterCallable::call(const Variant **p_arguments, int p_argcount, Va
 			"Resumed after await, but class instance is gone.");
 #endif
 
-	MonoArray *signal_args = nullptr;
-
-	if (p_argcount > 0) {
-		signal_args = mono_array_new(mono_domain_get(), CACHED_CLASS_RAW(MonoObject), p_argcount);
-
-		for (int i = 0; i < p_argcount; i++) {
-			MonoObject *boxed = GDMonoMarshal::variant_to_mono_object(*p_arguments[i]);
-			mono_array_setref(signal_args, i, boxed);
-		}
-	}
-
-	MonoObject *awaiter = awaiter_handle.get_target();
-
-	if (!awaiter) {
-		r_call_error.error = Callable::CallError::CALL_ERROR_INSTANCE_IS_NULL;
-		return;
-	}
-
 	MonoException *exc = nullptr;
-	CACHED_METHOD_THUNK(SignalAwaiter, SignalCallback).invoke(awaiter, signal_args, &exc);
+	bool awaiter_is_null = false;
+	GDMonoCache::cached_data.methodthunk_SignalAwaiter_SignalCallback.invoke(awaiter_handle.get_intptr(), p_arguments, p_argcount, &awaiter_is_null, &exc);
 
 	if (exc) {
 		GDMonoUtils::set_pending_exception(exc);
 		ERR_FAIL();
-	} else {
-		r_call_error.error = Callable::CallError::CALL_OK;
 	}
+
+	if (awaiter_is_null) {
+		r_call_error.error = Callable::CallError::CALL_ERROR_INSTANCE_IS_NULL;
+		return;
+	}
+
+	r_call_error.error = Callable::CallError::CALL_OK;
 }
 
-SignalAwaiterCallable::SignalAwaiterCallable(Object *p_target, MonoObject *p_awaiter, const StringName &p_signal) :
+SignalAwaiterCallable::SignalAwaiterCallable(Object *p_target, MonoGCHandleData p_awaiter_handle, const StringName &p_signal) :
 		target_id(p_target->get_instance_id()),
-		awaiter_handle(MonoGCHandleData::new_strong_handle(p_awaiter)),
+		awaiter_handle(p_awaiter_handle),
 		signal(p_signal) {
 }
 
@@ -152,7 +139,7 @@ bool EventSignalCallable::compare_equal(const CallableCustom *p_a, const Callabl
 		return false;
 	}
 
-	if (a->event_signal != b->event_signal) {
+	if (a->event_signal_name != b->event_signal_name) {
 		return false;
 	}
 
@@ -167,7 +154,7 @@ bool EventSignalCallable::compare_less(const CallableCustom *p_a, const Callable
 }
 
 uint32_t EventSignalCallable::hash() const {
-	uint32_t hash = event_signal->field->get_name().hash();
+	uint32_t hash = event_signal_name.hash();
 	return hash_murmur3_one_64(owner->get_instance_id(), hash);
 }
 
@@ -177,8 +164,7 @@ String EventSignalCallable::get_as_text() const {
 	if (script.is_valid() && script->get_path().is_resource_file()) {
 		class_name += "(" + script->get_path().get_file() + ")";
 	}
-	StringName signal = event_signal->field->get_name();
-	return class_name + "::EventSignalMiddleman::" + String(signal);
+	return class_name + "::EventSignalMiddleman::" + String(event_signal_name);
 }
 
 CallableCustom::CompareEqualFunc EventSignalCallable::get_compare_equal_func() const {
@@ -194,39 +180,38 @@ ObjectID EventSignalCallable::get_object() const {
 }
 
 StringName EventSignalCallable::get_signal() const {
-	return event_signal->field->get_name();
+	return event_signal_name;
 }
 
 void EventSignalCallable::call(const Variant **p_arguments, int p_argcount, Variant &r_return_value, Callable::CallError &r_call_error) const {
 	r_call_error.error = Callable::CallError::CALL_ERROR_INVALID_METHOD; // Can't find anything better
 	r_return_value = Variant();
 
-	ERR_FAIL_COND(p_argcount < event_signal->invoke_method->get_parameters_count());
-
 	CSharpInstance *csharp_instance = CAST_CSHARP_INSTANCE(owner->get_script_instance());
 	ERR_FAIL_NULL(csharp_instance);
 
-	MonoObject *owner_managed = csharp_instance->get_mono_object();
-	ERR_FAIL_NULL(owner_managed);
-
-	MonoObject *delegate_field_value = event_signal->field->get_value(owner_managed);
-	if (!delegate_field_value) {
-		r_call_error.error = Callable::CallError::CALL_OK;
-		return;
-	}
+	GCHandleIntPtr owner_gchandle_intptr = csharp_instance->get_gchandle_intptr();
 
 	MonoException *exc = nullptr;
-	event_signal->invoke_method->invoke(delegate_field_value, p_arguments, &exc);
+	bool awaiter_is_null = false;
+	GDMonoCache::cached_data.methodthunk_ScriptManagerBridge_RaiseEventSignal.invoke(
+			owner_gchandle_intptr, &event_signal_name,
+			p_arguments, p_argcount, &awaiter_is_null, &exc);
 
 	if (exc) {
 		GDMonoUtils::set_pending_exception(exc);
 		ERR_FAIL();
-	} else {
-		r_call_error.error = Callable::CallError::CALL_OK;
 	}
+
+	if (awaiter_is_null) {
+		r_call_error.error = Callable::CallError::CALL_ERROR_INSTANCE_IS_NULL;
+		return;
+	}
+
+	r_call_error.error = Callable::CallError::CALL_OK;
 }
 
-EventSignalCallable::EventSignalCallable(Object *p_owner, const CSharpScript::EventSignal *p_event_signal) :
+EventSignalCallable::EventSignalCallable(Object *p_owner, const StringName &p_event_signal_name) :
 		owner(p_owner),
-		event_signal(p_event_signal) {
+		event_signal_name(p_event_signal_name) {
 }