add impl for DigestPrimitive on keccak feature

[TheRealBluesun]

Abstract

This PR adds an impl DigestPrimitive when the feature keccak256 is used. This should resolve a build error when using the feature.

[tarcieri]

I think what you're looking for is k256::ecdsa::recoverable::Signature, which provides Ethereum-style recoverable signatures (which carry an extra byte as a "recovery ID"):

https://docs.rs/k256/0.7.1/k256/ecdsa/recoverable/index.html

Note that this signature type already has an impl equivalent to the one you're trying to add here:

https://github.com/RustCrypto/elliptic-curves/blob/80f33e8/k256/src/ecdsa/recoverable.rs#L259-L262

...also note that DigestSignature can only be impl'd once for any given signature type as it defines a "default" digest primitive, so the tests for this PR are failing due to those conflicting implementations.

If you really want to produce a standard (non-recoverable) ECDSA signature with Keccak256 (which I'm guessing isn't the case), the only way to do it is by passing a Keccak256 instance directly to DigestSigner.

[TheRealBluesun]

Thank you so much for taking the time to respond!

You are correct, this was my misunderstanding. Cheers!

[tarcieri]

Opened #270 to hopefully clarify the documentation