`convert_bits` Iterator

[TheBlueMatt]

When writing out an object as a bech32 string via fmt it seems we currently have to first encode the [u8] into a Vec<u5> and only then we can call encode_without_checksum_to_fmt. This is a bit wasteful, ISTM it wouldn't take a whole log of change to build a version of convert_bits that returns an iterator rather than a Vec, and allows avoiding the intermediate allocation.

[tcharding]

I think this has been resolved by the recent crate re-write, likely can be closed.

[apoelstra]

Yep, it has. Both the iterator API and the top-level API let you do this without any allocations.