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/godotsharp_dirs.cpp

@@ -96,8 +96,6 @@ class _GodotSharpDirs {
 public:
 	String res_data_dir;
 	String res_metadata_dir;
-	String res_assemblies_base_dir;
-	String res_assemblies_dir;
 	String res_config_dir;
 	String res_temp_dir;
 	String res_temp_assemblies_base_dir;
@@ -105,6 +103,9 @@ public:
 	String mono_user_dir;
 	String mono_logs_dir;
 
+	String api_assemblies_base_dir;
+	String api_assemblies_dir;
+
 #ifdef TOOLS_ENABLED
 	String mono_solutions_dir;
 	String build_logs_dir;
@@ -113,7 +114,6 @@ public:
 	String csproj_filepath;
 
 	String data_editor_tools_dir;
-	String data_editor_prebuilt_api_dir;
 #else
 	// Equivalent of res_assemblies_dir, but in the data directory rather than in 'res://'.
 	// Only defined on export templates. Used when exporting assemblies outside of PCKs.
@@ -131,8 +131,6 @@ private:
 	_GodotSharpDirs() {
 		res_data_dir = ProjectSettings::get_singleton()->get_project_data_path().plus_file("mono");
 		res_metadata_dir = res_data_dir.plus_file("metadata");
-		res_assemblies_base_dir = res_data_dir.plus_file("assemblies");
-		res_assemblies_dir = res_assemblies_base_dir.plus_file(GDMono::get_expected_api_build_config());
 		res_config_dir = res_data_dir.plus_file("etc").plus_file("mono");
 
 		// TODO use paths from csproj
@@ -140,6 +138,8 @@ private:
 		res_temp_assemblies_base_dir = res_temp_dir.plus_file("bin");
 		res_temp_assemblies_dir = res_temp_assemblies_base_dir.plus_file(_get_expected_build_config());
 
+		api_assemblies_base_dir = res_data_dir.plus_file("assemblies");
+
 #ifdef JAVASCRIPT_ENABLED
 		mono_user_dir = "user://";
 #else
@@ -169,7 +169,7 @@ private:
 
 		String data_dir_root = exe_dir.plus_file("GodotSharp");
 		data_editor_tools_dir = data_dir_root.plus_file("Tools");
-		data_editor_prebuilt_api_dir = data_dir_root.plus_file("Api");
+		api_assemblies_base_dir = data_dir_root.plus_file("Api");
 
 		String data_mono_root_dir = data_dir_root.plus_file("Mono");
 		data_mono_etc_dir = data_mono_root_dir.plus_file("etc");
@@ -189,8 +189,8 @@ private:
 			data_editor_tools_dir = exe_dir.plus_file("../Resources/GodotSharp/Tools");
 		}
 
-		if (!DirAccess::exists(data_editor_prebuilt_api_dir)) {
-			data_editor_prebuilt_api_dir = exe_dir.plus_file("../Resources/GodotSharp/Api");
+		if (!DirAccess::exists(api_assemblies_base_dir)) {
+			api_assemblies_base_dir = exe_dir.plus_file("../Resources/GodotSharp/Api");
 		}
 
 		if (!DirAccess::exists(data_mono_root_dir)) {
@@ -234,6 +234,8 @@ private:
 #endif
 
 #endif
+
+		api_assemblies_dir = api_assemblies_base_dir.plus_file(GDMono::get_expected_api_build_config());
 	}
 
 public:
@@ -251,14 +253,6 @@ String get_res_metadata_dir() {
 	return _GodotSharpDirs::get_singleton().res_metadata_dir;
 }
 
-String get_res_assemblies_base_dir() {
-	return _GodotSharpDirs::get_singleton().res_assemblies_base_dir;
-}
-
-String get_res_assemblies_dir() {
-	return _GodotSharpDirs::get_singleton().res_assemblies_dir;
-}
-
 String get_res_config_dir() {
 	return _GodotSharpDirs::get_singleton().res_config_dir;
 }
@@ -275,6 +269,14 @@ String get_res_temp_assemblies_dir() {
 	return _GodotSharpDirs::get_singleton().res_temp_assemblies_dir;
 }
 
+String get_api_assemblies_dir() {
+	return _GodotSharpDirs::get_singleton().api_assemblies_dir;
+}
+
+String get_api_assemblies_base_dir() {
+	return _GodotSharpDirs::get_singleton().api_assemblies_base_dir;
+}
+
 String get_mono_user_dir() {
 	return _GodotSharpDirs::get_singleton().mono_user_dir;
 }
@@ -303,10 +305,6 @@ String get_project_csproj_path() {
 String get_data_editor_tools_dir() {
 	return _GodotSharpDirs::get_singleton().data_editor_tools_dir;
 }
-
-String get_data_editor_prebuilt_api_dir() {
-	return _GodotSharpDirs::get_singleton().data_editor_prebuilt_api_dir;
-}
 #else
 String get_data_game_assemblies_dir() {
 	return _GodotSharpDirs::get_singleton().data_game_assemblies_dir;