Further changes in GDNative API

- Added new_copy to all types, since trivial copy won't work for all
  types.
- Added functions to convert from String to char array types, which is
  not provided by the methods bound in Variant.
- Added operator index to String.
- Added missing cstring version of some Variant functions. They existed
  in the header but didn't have the implementation and were missing from
  the gdnative_api.json file.
- Added support for static calls on Variant types.

modules/gdnative/gdnative/string_name.cpp

@@ -44,9 +44,7 @@ void GDAPI godot_string_name_new(godot_string_name *r_dest) {
 }
 
 void GDAPI godot_string_name_new_copy(godot_string_name *r_dest, const godot_string_name *p_src) {
-	StringName *dest = (StringName *)r_dest;
-	const StringName *src = (const StringName *)p_src;
-	memnew_placement(dest, StringName(*src));
+	memnew_placement(r_dest, StringName(*(StringName *)p_src));
 }
 
 void GDAPI godot_string_name_new_with_latin1_chars(godot_string_name *r_dest, const char *p_contents) {