Now that we always generate an extra proper VerificationKey for each
Tweak::ChangePubkey case, this /should/ never fail: it also helps split out the
actual verification of the signature from the parsing and validation of the key
itself.
* Implement Expanded to Compact Difficulty
* Implement Arbitrary for CompactDifficulty
Remove the derive, and generate values from random block
hashes.
* Implement Arbitrary for ExpandedDifficulty and Work
* Use Arbitrary for CompactDifficulty in Arbitrary for Block
* Test difficulty on all block test vectors
And cleanup some duplicate test code
* Round-trip tests for compact test cases
* Round-trip tests for compact difficulty in block test vectors
* Make Add for Work return PartialCumulativeWork
Remove AddAssign for Work
Rewrite a proptest using Sub for PartialCumulativeWork
Use Arbitrary for Work
* Add roundtrip work sum tests
* Add roundtrip comparison difficulty tests
* Add failing proptest cases due to test bugs
* Use Some(_) rather than _.into()
* Reduce visibility of difficulty type inner values
* Split work and other difficulty proptests
This change makes sure that rejected work values don't disable property
tests on other types.