signature: tracking issue for derive(Signer) and derive(Verifier) stabilization

[tarcieri]

The signature crate contains (as re-exports from the signature_derive crate) the following procedural macros which are presently gated under the derive-preview feature:

• derive(Signer) - support for deriving Signature on types which implement DigestSignature for signature types which are also marked as DigestSignature
• derive(Verifier) - support for deriving Verifier on types which implement DigestVerifier for signature types which are also marked as DigestSignature

The main blocker for stabilization of this feature is #92: stabilizing the other digest-related functionality the proc macro builds on, and ongoing design questions.

[tarcieri]

We landed on a proc macro for this purpose after a lot of experimentation with alternative approaches, including blanket impls w\ marker traits. Some past discussion can be found here:

• RustCrypto/signatures#9: Add UseDigestTo* marker trait and blanket Sign/Verify impls
• RustCrypto/signatures#10: Add "raw digest" signature marker trait and sign/verify traits
• RustCrypto/signatures#12: Digestable signatures
• RustCrypto/signatures#18: signature_derive: Custom derive support for Signer/Verifier
• RustCrypto/signatures#27: Replace DigestSignature with derive attr
• RustCrypto/signatures#33: Revert removal of DigestSignature

The tl;dr: of all of these PRs is we've explored a lot of the potential design space and alternatives to a proc macro, netting out where we are because none of them were flexible enough to cover all of the potential use cases.

[tarcieri]

Stabilized in #1141