Reduce unnecessary COW on Vector by make writing explicit

This commit makes operator[] on Vector const and adds a write proxy to it.  From
now on writes to Vectors need to happen through the .write proxy. So for
instance:

Vector<int> vec;
vec.push_back(10);
std::cout << vec[0] << std::endl;
vec.write[0] = 20;

Failing to use the .write proxy will cause a compilation error.

In addition COWable datatypes can now embed a CowData pointer to their data.
This means that String, CharString, and VMap no longer use or derive from
Vector.

_ALWAYS_INLINE_ and _FORCE_INLINE_ are now equivalent for debug and non-debug
builds. This is a lot faster for Vector in the editor and while running tests.
The reason why this difference used to exist is because force-inlined methods
used to give a bad debugging experience. After extensive testing with modern
compilers this is no longer the case.

modules/mono/editor/bindings_generator.cpp

@@ -1650,7 +1650,7 @@ Error BindingsGenerator::_generate_glue_method(const BindingsGenerator::TypeInte
 								   "\t\tvarargs.set(i, GDMonoMarshal::mono_object_to_variant(elem));\n"
 								   "\t\t" C_LOCAL_PTRCALL_ARGS ".set(");
 				p_output.push_back(real_argc_str);
-				p_output.push_back(" + i, &varargs[i]);\n\t" CLOSE_BLOCK);
+				p_output.push_back(" + i, &varargs.write[i]);\n\t" CLOSE_BLOCK);
 			} else {
 				p_output.push_back(c_in_statements);
 				p_output.push_back("\tconst void* " C_LOCAL_PTRCALL_ARGS "[");