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.

editor/plugins/sprite_editor_plugin.cpp

@@ -169,7 +169,7 @@ void SpriteEditor::_update_mesh_data() {
 
 	Size2 img_size = Vector2(image->get_width(), image->get_height());
 	for (int j = 0; j < lines.size(); j++) {
-		lines[j] = expand(lines[j], rect, epsilon);
+		lines.write[j] = expand(lines[j], rect, epsilon);
 
 		int index_ofs = computed_vertices.size();