Confusion over `pkcs1v15::VerifyingKey::new` vs `::new_with_prefix`

[tarcieri]

In #234 we had some people confused about signature verification failing with ::new, and they had trouble discovering ::new_with_prefix.

It would be good to either improve the documentation here, or eliminate the distinction between ::new and ::new_with_prefix and automatically determining if the prefix is needed (e.g. based on the OID of the provided digest).

It seems like other RSA libraries are able to make this determination automatically.

[dignifiedquire]

I think it would be great if we had

• new() -> determines it automatically
• new_prefix(p: Prefix) explicitly expects a prefix or not. With enum Prefix { Yes, No } (names can be bikeshedded) for when you want to enforce one or the other.

[tarcieri]

I think part of the rationale for the current split is avoiding the need to have an AssociatedOid on the digest used. However, that just seems to be causing confusion.

Looking at RFC8017 Appendix 2.4, I'm unclear under what circumstances the OID can actually be omitted?

So perhaps the current new should be renamed to something like new_unprefixed, and new_with_prefixed should be renamed to new?