[backport#15139] Replace use of BEGIN and END macros on uint256

Summary:
Replace use of BEGIN and END macros on uint256 with begin() and
end() methods in the Merkle tree code.

https://github.com/bitcoin/bitcoin/pull/15139/commits/7fa238c701b5b6d4f78b92334e38fac466ec0eb2

Partial backport of Core PR15139

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6288