Update to signature::DigestSigner/Verifier changes

[daxpedda]

Companion PR to RustCrypto/traits#2004.

[daxpedda]

Looking into how this would work for ML-DSA, because it uses a custom SHAKE wrapper internally that doesn't implement Digest.

[daxpedda]

So the wrapper isn't necessary to compute µ, we can use Shake256 for D here.
The problem is just that DigestSigner requires Digest, so what do you think about removing that requirement?
Individual implementations can still require it separately.

[tarcieri]

@daxpedda the only digest-related functionality that's really needed by the caller now is Update, so you can change it to that

[tarcieri]

@daxpedda want to try it out with ml-dsa external mu or save that for another PR?

[daxpedda]

Working on it right now.
Just adding a test actually.

[daxpedda]

Ideally, we should just call a method on SigningKey here so we don't duplicate the internals.

I could change SigningKey::raw_sign_deterministic() to take a Fn instead as well.

[daxpedda]

So I did try this in daxpedda@2c77bab.
I believe my attempt ended up being quite awful.

[tarcieri]

@daxpedda could you extract some methods, one to do setup and one to do finalization? Kind of like what you proposed in RustCrypto/traits#2004 but to abstract over the internals for computing Shake256

[daxpedda]

On it.

[daxpedda]

Done in #1073.

[tarcieri]

@daxpedda mind doing a PR for https://github.com/RustCrypto/elliptic-curves too?

[daxpedda]

@daxpedda mind doing a PR for https://github.com/RustCrypto/elliptic-curves too?

Will do!