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/gdnative/arvr/arvr_interface_gdnative.cpp

@@ -125,7 +125,7 @@ bool ARVRInterfaceGDNative::is_stereo() {
 	return stereo;
 }
 
-bool ARVRInterfaceGDNative::is_initialized() {
+bool ARVRInterfaceGDNative::is_initialized() const {
 	bool initialized;
 
 	ERR_FAIL_COND_V(interface == NULL, false);