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.

rust-dirstate: provide CopyMapIter and StateMapIter types They will be used in the declaration of Python iterator types.

rust-dirstate: specify concrete return type of DirsMultiset::iter() This allows us to put a returned iterator in a struct. We could implement DirsMultisetIter(hash_map::Keys<..>) struct to hide the implementation detail, but I think type alias is good enough for us.