Replace use of `BEGIN` and `END` macros on uint256 with `begin()` and
`end()` methods in the Merkle tree code.
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https://github.com/bitcoin/bitcoin/pull/15139/commits/7fa238c701b5b6d4f78b92334e38fac466ec0eb2
Partial backport of Core [[https://github.com/bitcoin/bitcoin/pull/15139 | PR15139]]