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/io/pck_packer.cpp

@@ -120,7 +120,7 @@ Error PCKPacker::flush(bool p_verbose) {
 	for (int i = 0; i < files.size(); i++) {
 
 		file->store_pascal_string(files[i].path);
-		files[i].offset_offset = file->get_position();
+		files.write[i].offset_offset = file->get_position();
 		file->store_64(0); // offset
 		file->store_64(files[i].size); // size