fa99e108e7 Fix implicit-integer-sign-change in arith_uint256 (MarcoFalke)
Pull request description:
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.
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Tree-SHA512: f227e2fd22021e39f0445ec041f4a299d13477c23cef0fc06c53fb3313cbe550cec329336224a7e8775d9045b8009423052b394e83d42a1e40772085dfcdd471