split: use literal syntax to build a set of one element

rust-cpython: leverage py_shared_iterator::from_inner() where appropriate

rust-cpython: remove Option<_> from interface of py_shared_iterator It's the implementation detail of the py_shared_iterator that the leaked reference is kept in Option<_> so that it can be dropped early.

rust-cpython: rename py_shared_iterator_impl to py_shared_iterator It's a public interface now.

rust-cpython: replace dyn Iterator<..> of mapping with concrete type See the previous commit for why. The docstring is moved accordingly.

rust-cpython: replace dyn Iterator<..> of sequence with concrete type We wouldn't care the cost of the dynamic dispatch, but I feel a concrete type helps understanding error messages.