Fix implicit-integer-sign-change in arith_uint256

Summary:
This refactor doesn't change behaviour, but clarifies that the numbers being dealt with aren't supposed to be negative. This helps when reading the code and allows to remove a sanitizer suppression for the whole file.

This is a backport of core#24059

Depends on D12813

Test Plan:
With UBSAN
ninja && ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12814