Added
- Support for methods of built-in types.
- `refdict({}).keys` is `dict.keys`.
- `refdict([]).append` is `list.append`.
- Keys of operator `[]` & `[]=` can be `int` and `slice`.
- Support for simple containment check.
- Support for `str()`.
- Support for item's chain deletion.
- `rd=refdict({'a':{'b':123}});del rd[a.b] => rd=refdict({'a':{}})`.