Derive Macro signature::Signer

#[derive(Signer)]

This is supported on feature="derive-preview" only.

Derive the Signer trait for a type which impls DigestSigner.

When implementing the DigestSigner trait for a signature type which itself impl's the [DigestSignature] trait (which marks signature algorithms which are computed using a Digest), signature providers can automatically derive the Signer trait when the digest algorithm is [DigestSignature::Digest] (i.e. the "standard" digest algorithm for a given signature type)

This automates all of the digest computation otherwise needed for a complete signature algorithm implementation.