feat/verification enum

[aagbotemi]

Summary

BIP-322 defines three validator outputs: valid at time T and age S, inconclusive, and invalid. Two of those lived in Verification while the third was reported as Err, so the spec's own state machine was split across a Rust idiom. Verification now carries all three.

Changes

• Added Invalid(Error) variant to Verification.
• Added a Box around TransactionExtract's source.
• Removed #[allow(clippy::result_large_err)] from every function.

Notes for reviewers

Invalid carries the full Error, which means Verification can no longer derive Clone, Copy, PartialEq or Eq. Error implements none of them, and bitcoin::io::Error can't.

Closes #83
Closes #58

[raphjaph]

This looks good overall. I just merged the changes from #77. A couple thoughts for you since you are a consumer of this library:

1. create_to_sign and extract_tx are more internal integrity failures rather than Verifiction::Invalid so maybe they keep their error variants? I don't feel strongly about this and the way it is right now seems okay to me.

2. check_to_sign only ever produces Error::ToSignInvalid so maybe we don't need the Option there?

3. This is an existing footgun I can see being problematic: calling .is_ok() doesn't necessarily mean the proof is valid, just that the input decoded valid. It can be Ok(Verification::Invalid(..)). We could either flatten everything to just return Verification or add some sort of #[must_use] + a helper .is_valid() + a note in the Changelog about this.

Let me know your thoughts.