[Chronik] Move HashToArray to util/hash.cpp/h

Summary: This is useful in e.g. chronik_validationinterface.cpp, and having it in chronik_bridge.cpp/h is a bit out of place.

Test Plan: ninja && ninja check

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12658