Make TxToUniv take a BlockHash instead of a raw uint256

Summary:
This enforces strict typing and makes the intent more clear.
There is no change in behavior.

Test Plan:

ninja all check-all
ninja bitcoin-fuzzers

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12429