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/code_editor.cpp

@@ -725,7 +725,7 @@ void CodeTextEditor::_complete_request() {
 	int i = 0;
 	for (List<String>::Element *E = entries.front(); E; E = E->next()) {
 
-		strs[i++] = E->get();
+		strs.write[i++] = E->get();
 	}
 
 	text_editor->code_complete(strs, forced);