* Add `Context` type
Adds a generic type which can be used with `SigningKey` and
`VerifyingKey` for storing a context string value along with the key for
use with `DigestSigner` and `DigestVerifier`.
* Added Context tests, docs, and re-exports
* Added docs about SHA-512 for prehashing; re-re-exported Sha512
Co-authored-by: Tony Arcieri <bascule@gmail.com>
Co-authored-by: Michael Rosenberg <michael@mrosenberg.pub>
