BytesKeyEC2: Prepare curve validation

[chrysn]

Contributes-To: #93

A first step is to go from a type alias (which the enum CredentialKey can make no guarantees on, because pub enum variant members are always pub) into a newtype.

This is a breaking API change, so maybe we should prioritize doing the API change even when we can't validate, but this will need a bit more than this single commit.

[chrysn]

Open points:

• I'd consider shedding a few derived traits:

  • PartialEq is necessary because CredentialKey has it too. Is this relevant outside of testing? (Doesn't hurt too much, either).
  • I'm conflicted about the try-from-slice -- it's convenient transitionally because most creation happened from slices anyway, but … dunno.

• How do we do the verification? When the type is created, that's typically far down in the parsing step, and we don't have access to a Cryptography.

[chrysn]

More precise open question: Should we even be doing this on BytesKeyEC2, or should that not be BytesP256ElemLen that would become a coordinate type?