python-codex32 reference impl

[BenWestgate]

apoelstra/rust-codex32#2 (comment):

@BenWestgate if you have a rewrite feel free to open a PR -- if it's not too hard to review I'm happy to take it in.

But I think the correct direction to rewrite in is one where we add a rust-bech32 dependency, use that for all the checksumming and encoding stuff, and then here we add (a) utility methods, and (b) constructors and accessors for the id/threshold/share index.

I have wrote this python implementation that covers more tests than the rust one does. It also does the approach above and relies on general checksum and bech32 modules. So it also passes Bech32/Bech32m tests.

It includes new test vectors for bitcoin/bips#2040 and bitcoin/bips#2077.

From: https://github.com/BenWestgate/python-codex32

Published on PyPI: https://pypi.org/project/codex32/

[apoelstra]

I vote to merge this with a cursory review; later when I improve rust-codex32, I will steal all the test vectors from this implementation, and presumably if they pass on both implementations, they're both correct :P.

cursory review ACK 4857e18 -- checked that this is shaped like it does codex32 stuff, that there's nothing obviously malicious, no filesystem or network access, etc.

cc @roconnor-blockstream can you let me know if I should hold off on merging this? Fine to ping me out-of-band.