Now, python implementation has a private _impl member, which is shared between dicts, proxies, and their views.

Also, MultiDict and MultiDictProxy are both derived from a private class _Base.

Pure Python implementation should be reworked to strictly follow C version:

1. MultiDict / CIMultiDict are self-contained, they don't expose any implementation details to other classes.
2. MultiDictProxy / CIMultiDictProxy has the only private member referencing a multidict instance. No _impl sharing anymore.
3. views should reference a multidict, not ._impl.