No support for ECDSA P-256

[SegalMan]

I'd like to open a PR supporting ECDSA P-256 at least for verification as that is my current need (already using the library with an ed25519 and now having to add a key using this algorithm)

The only main decisions I see from my perspective in that regard:

1. Which create should be used (leading candidates are p256 and ecdsa)?
2. The FailedToVerify error currently having an associated ed25519_dalek::SignatureError, meaning I'd need to either introduce a separate error for verification failure for ECDSA or introduce breaking changes to that error

I didn't want to overstep and directly open a PR, so let's discuss :)