[backport#17261 9/13] refactor: define a UINT256_ONE global constant

Summary:
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 PR17261

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7858