rust-pyo3-sharedref: macro to define Python iterators This convenience will help reducing the boilerplate for one of the most common use cases of the `pyo3_sharedref` crate. It is an improved adaptation of a similar macro that was in `hg-cpython` (but not in `rust-cpython`). The improvement is that it takes care of the constructor, sparing the `share_map` to the caller.