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/array.cpp

@@ -43,6 +43,10 @@ void GDAPI godot_array_new(godot_array *p_self) {
 	memnew_placement(p_self, Array);
 }
 
+void GDAPI godot_array_new_copy(godot_array *r_dest, const godot_array *p_src) {
+	memnew_placement(r_dest, Array(*(Array *)p_src));
+}
+
 void GDAPI godot_array_destroy(godot_array *p_self) {
 	((Array *)p_self)->~Array();
 }