All implementations of `radicle_crypto::Signer` are also
`signature::Signer`. Also, the implementations of `Signer::sign` and
`Signer::try_sign` call each other.
To simplify, make `signature::Signer` a requirement for
`radicle_crypto::Signer`.
Since all signers implement `signature::Keypair`, we can simplify the
implementations of `signature::Signer<ExtendedSignature>`. A blanket
implementation is not possible, because we do not control
`signature::Signer`.
The various signers defined in `radicle-crypto` all allow borrowing
the corresponding public key a.k.a. "verifying key". This is captured
by implementing `signature::KeypairRef`, so implement it in addition
to `signature::Signer`.
Lorenz Leutgeb