[Chronik] Add TxId::new and TxId::hash

Summary:
Helpers when dealing with txids.

Depends on D15007.

Test Plan: cargo test -p bitcoinsuite-core

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15010