async-signature: remove AsyncDigestSigner blanket implementation

[baloo]

This introduces a basic check to make sure our blanket implementation would not block manual implementation of the traits in this crate.

Before this commit AsyncDigestSigner was causing a conflict:

  --> async-signature/tests/mock_impl.rs:14:1
   |
14 | / impl<D> async_signature::AsyncDigestSigner<D, Signature> for MockSigner
15 | | where
16 | |     D: async_signature::Digest,
   | |_______________________________^
   |
   = note: conflicting implementation in crate `async_signature`:
           - impl<D, S, T> AsyncDigestSigner<D, S> for T
             where D: Digest, T: DigestSigner<D, S>;
   = note: downstream crates may implement trait `async_signature::signature::DigestSigner<_, Signature>` for type `MockSigner`

[tarcieri]

Hmm, that's unfortunate. I wonder if we should reconsider the other blanket impls as well.

[tarcieri]

@baloo perhaps we should bump the version number as well? I think this is a technically a breaking change in that someone could previously use a DigestSigner as an AsyncDigestSigner

[baloo]

I think other blankets were fine (I believe I tested them here, but they might still cause issues, I don't know).

This is a breaking change. I'll submit a follow-up PR to bump it.
I think the only consumers for now are in formats.git and those are not tagged yet (so easy to bump)

[baloo]

Actually, the async-signature is currently sitting at 0.6.0-pre.1, I think I'm alright with such an API break in the next -pre or -rc release.

[tarcieri]

Aah right! Seems good then