refactor: Improve encapsulation between MuHash3072 and Num3072

Summary:
This is a backport of core#19145 [1/3]
https://github.com/bitcoin/bitcoin/pull/19145/commits/a1fcceac69097a8e6540a6fd8121a5d53022528f

Depends on D11150

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11155