Instead of having a uint256 representations of one scattered throughout
where it is used, define it globally in uint256.h
https://github.com/bitcoin/bitcoin/pull/17261/commits/4977c30d59e88a3e5ee248144bcc023debcd895b
Depends on D7855
Partial backport of Core [[https://github.com/bitcoin/bitcoin/pull/17261 | PR17261]]