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/animation_blend_space_2d_editor.cpp

@@ -414,7 +414,7 @@ void AnimationNodeBlendSpace2DEditor::_blend_space_draw() {
 			point = (point - blend_space->get_min_space()) / (blend_space->get_max_space() - blend_space->get_min_space());
 			point *= s;
 			point.y = s.height - point.y;
-			points[j] = point;
+			points.write[j] = point;
 		}
 
 		for (int j = 0; j < 3; j++) {