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/find_in_files.cpp

@@ -143,7 +143,7 @@ void FindInFiles::_iterate() {
 
 		// Scan folders first so we can build a list of files and have progress info later
 
-		PoolStringArray &folders_to_scan = _folders_stack[_folders_stack.size() - 1];
+		PoolStringArray &folders_to_scan = _folders_stack.write[_folders_stack.size() - 1];
 
 		if (folders_to_scan.size() != 0) {
 			// Scan one folder below