rust-cpython: remove useless Option<$leaked> from py_shared_iterator...
rust-cpython: remove useless Option<$leaked> from py_shared_iterator
We no longer need to carefully drop the iterator when it's consumed. Mutation
is allowed even if the iterator exists.
There's a minor behavior change: next(iter) may return/raise something other
than StopIteration if it's called after the iterator has been fully consumed,
and if the Rust object isn't a FusedIterator.