To get counts of items before getting the list, which is useful for
GDNative so users can pre-allocate the buffer with the correct size
without having to get the list twice.
Copied relevant documentation from the original `@GDScript` built-ins,
which will likely be removed in a future commit.
Various fixups to `variant_utility.cpp` while working on this.
* Using C-style function pointers now, InternalMethod is gone.
* This ensures much better performance in typed code.
* Renamed builtin_funcs to utility_funcs, to avoid naming confusion
