Fix the UsageFlags structure

[hug-dev]

We added a clarification in the book in parallaxsecond/parsec-book#110 regarding sign/verify_hash/message.

The following part is specifically important about sign/verify_hash:

This flag automatically sets sign_message: if an application sets the flag sign_hash when creating a key, then the key always has the permissions conveyed by sign_message. For a key pair, this concerns the private key.

I see multiple ways to fix this:

1. Make the fields of UsageFlags private (or just the sign/verify ones?) with setters and getters on the different usages. set_sign_hash would set both sign_hash and sign_message to true.
2. Just add verify/sign_message methods that return true if either sign/verify_message or sign/verify_hash are true.

I would prefer 1 I think but that is a breaking change. That's probably fine. 2 would make the behaviour correct if you use the new method but we would have to check that we use that everywhere. It's not breaking though.