In order to respond to the caller with the correct feature level in
case sigrefs are to be saved but unchanged, the write side needs to
care about feature levels. This is done by sharing more code with the
read side.
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