[minor API BREAK] Add unit tests to cover all error cases

This comes with a couple bugfixes and the following API changes:

  - Secp256k1::sign and ::sign_compact no longer return Result;
    it is impossible to trigger their failure modes with safe
    code since the `Message` and `SecretKey` types validate when
    they are created.

  - constants::MAX_COMPACT_SIGNATURE_SIZE loses the MAX_; signatures
    are always constant size

  - the Debug output for everything is now hex-encoded rather than
    being a list of base-10 ints. It's just easier to read this way.

kcov v26 now reports 100% test coverage; however, this does not
guarantee that test coverage is actually complete. Patches are
always welcome for improved unit tests.