Add a facility to create arith_uint256 from their double representation

Summary: This will make it easier to do computation on hash targets.

Test Plan:

ninja check-bitcoin-arith_uint256_tests

Reviewers: #bitcoin_abc, PiRK, roqqit

Reviewed By: #bitcoin_abc, PiRK, roqqit

Differential Revision: https://reviews.bitcoinabc.org/D16575