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.

servers/visual_server.cpp

@@ -744,7 +744,7 @@ Error VisualServer::_surface_set_data(Array p_arrays, uint32_t p_format, uint32_
 
 		if (first) {
 			for (int i = 0; i < total_bones; i++) {
-				r_bone_aabb[i].size = Vector3(-1, -1, -1); //negative means unused
+				r_bone_aabb.write[i].size = Vector3(-1, -1, -1); //negative means unused
 			}
 		}