Improve naming consistency in the C wrapper

[paulhowardarm]

Summary

The current state of the C wrapper is a bit mixed up with regards to naming conventions. Since C is not a namespaced language, it's important to use prefixes and use them consistently. We need to do a pass over the current C wrapper and make sure we have a consistent approach.

Details

We can debate exactly what should change to what, but here are some places where we probably need to make some kind of fix:

• The members of the VeraisonResult enum probably should have a prefix, eg VeraisonOk rather than just Ok
• The ChallengeResponseSession structure probably should have a prefix, and maybe its casing style is also not very C-like - can the cbindgen tool do on-the-fly renaming to better match the typical casing conventions of C interfaces?
• The functions open_challenge_response_session, challenge_response, free_challenge_response_session all need a veraison_ prefix.
• The challenge_response function arguably could do with a more active name, such as veraison_challenge_response_supply_evidence