halfagg: Add Python specification and extensive test vectors

[fjahr]

Part of an effort to get the halfagg BIP ready for a pull to the bips repo.

This PR intends to resolve the following pending TODOs:

Test latest version of hacspec (run through checker)

Instead this PR adds a Python specification. The problem I have run into is that since the specification code was written hacspec (the implementation language) has stopped development and was superseeded by hax which also implements hacspec (the specification language) follows a different philosophy than hacspec (implementation language). Hax is under active development and could still have bigger changes and documentation is mostly a work in progress. As a first step I ported the BIP340 example that was a dependency of the halfagg hacspec code to hax, that PR has not received any significant review in the last 6 months despite me reaching out to contributors. It simply isn't a big priority on their side. I even have a hax halfagg code that ran with an earlier version but it's really just working rust and I don't know if it's acceptable hax.

It seems to me that using Python is probably the best option, given that it has been using in many comparable BIPs and the advent of the secp256k1lab library has made this even more practical than it used to be.

The alternative would be to keep using the rust code that is there now, maybe updating it with a few tweaks. But I think trying to make it compatible with hax would take too long due to the lack of feedback and we would also run the risk of depending on an outdated hax version quite quickly. It would be practical to just have a specification in rust instead of Python but I would prefer to go with Python.

The Python code still has some very verbose comments that link the Python code closely to the pseudo code specification which is not the usual style for these kinds of specifications. I tend to think it should be removed but wanted to hear feedback is someone thinks differently. It was helpful for me to relate the BIP draft to the code but it's probably not something everyone is interested in.

Add failing verification test vectors that exercise edge cases.

Add aggregation test vectors (passing and failing, including edge cases)

This add a generator script which includes the tests that were part of the rust test code and adds extensive additional test vectors which are output as CSV files. There is also a runner script. This pattern follows that of several other comparable BIPs. I ran coverage on the vectors runner and it shows that every line of the Python specification is covered except for the raise in Aggregate which is unreachable if the VerifyAggregate code is implemented correctly.

Note that the max_count question is not addressed here and there are still tests added for the current value 2^16 - 1. That topic is probably best discussed in the existing PRs on this question.