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.

core/node_path.cpp

@@ -427,7 +427,7 @@ NodePath::NodePath(const String &p_path) {
 
 				String name = path.substr(from, i - from);
 				ERR_FAIL_INDEX(slice, data->path.size());
-				data->path[slice++] = name;
+				data->path.write[slice++] = name;
 			}
 			from = i + 1;
 			last_is_slash = true;