Proof of work related refactor

Continues #3839
Deleting this conversion won't hurt. GetCompact + SetCompact will only round the (absolute value of a) uint256 down, so the check becomes stricter. As it's only a heuristic, loosening it is never a problem. Given that all it does is divide by a power of 4, I expect it to be no-op apart from the last 2 bits anyway.

In short: the test is probably immeasurably stronger by having the conversion.