Some functions return a new instance of such containers,
but instead we made a copy of them, without taking ownership of the
original created by the function.
Now we use a specific constructor taking ownership on the godot_* struct.
Changed error message macros to actually use Godot's error reporting
facilities instead of outputting straight to stderr. This enables
GDNative errors to actually show up inside the editor.
Messages and set of available macros now also better matches that of
the engine itself.
