[Chronik] Add Hashed::digest + implementations

Summary:
Currently, the different implementations of Hashed are just shells of the hash but don't actually represent the given hash.

By adding digest to Hashed + implementations, we can actually compute the hashes.

Test Plan: ninja check-crates

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13946