Cast dict to mapping and list to sequence, transferring ownership

[davidhewitt]

We have PyDict::as_mapping, PyList::as_sequence, and PyTuple::as_sequence.

With the new Bound API, ownership matters, so we might want to add versions of these methods which transfer ownership.

• PyDict::into_mapping
• PyList::into_sequence
• PyTuple::as_sequence

maybe?

(The equivalent alternative at the moment is to use the unsafe .downcast_into_unchecked::<Mapping>() etc, but that also runs up against the friction observed with deref & ownership in #3980)