Modify extract adaptor algorithm

[siv2r]

Fixes #10 Fixes #14 Fixes #15

I've rebased this pull request on top of #12. Please merge #12 before merging this one.

[siv2r]

./tests.sh fails for me:

Yes, these fails happen in the test_pre_sign_generation (), test_pre_sign_nonce (), and test_pre_sign_nonce_without_auxrand() functions. I planned to fix this in another PR because these functions need other fixes, too (variable renaming, removal of #type ignore).

Alternatively, we can remove these functions completely:

• These tests are incomplete (no adapt, extract secadaptor, or adaptor tests)
• Other BIPs (327, 340) don’t implement these types of tests

So, removing them seems like a simple fix without significant loss.

[jonasnick]

Alternatively, we can remove these functions completely:

If we'd remove all three tests then we wouldn't have a randomized end-to-end test anymore. But maybe we can merge the three tests into one, since they seem to be testing similar things.