signature: tracking issue for `digest` stabilization (DigestSigner, DigestVerifier, and DigestSignature)

[tarcieri]

The signature crate contains the following traits which are presently gated under the digest-preview feature:

• DigestSignature - marker trait for signature algorithms based on a simple "prehash" structure. This is used in conjunction with custom derive support (signature: tracking issue for derive(Signer) and derive(Verifier) stabilization #93)
• DigestSigner - signer which takes a Digest of a message to be signed
• DigestVerifier - verifier which takes a Digest of a message to be verified

The main blocker for stabilization is a 1.0 release of the digest crate, but also some lingering design questions.

[tarcieri]

One issue discussed in #78 (and previously discussed in RustCrypto/signatures#10) is the naming of DigestSignature. It might be clearer to name it something like PrehashSignature, as its purpose is to act as a marker with a digest algorithm for signature algorithms which use a prehashing step when computing a signature.

Also noted was that the docs should be clarified that while prehashing is useful for Fiat-Shamir-based signature algorithms, there are such algorithms which don't utilize a prehashing step.

[daira]

The name PrehashSignature makes sense to me.