Merge pull request #5490

Merge pull request #5490

6bd0dc2 arith_uint256: remove initialization from byte vector (Wladimir J. van der Laan)
30007fd Remove now-unused methods from arith_uint256 and base_uint (Wladimir J. van der Laan)
edc7204 Remove arith_uint160 (Wladimir J. van der Laan)
dba2e91 Add tests for new uint256 (Wladimir J. van der Laan)
92cdb1a Add conversion functions arith_uint256<->uint_256 (Wladimir J. van der Laan)
bfc6070 uint256->arith_uint256 blob256->uint256 (Wladimir J. van der Laan)
734f85c Use arith_uint256 where necessary (Wladimir J. van der Laan)
34cdc41 String conversions uint256 -> uint256S (Wladimir J. van der Laan)
2eae315 Replace uint256(1) with static constant (Wladimir J. van der Laan)
8076585 Replace GetLow64 with GetCheapHash (Wladimir J. van der Laan)
4f15249 Replace direct use of 0 with SetNull and IsNull (Wladimir J. van der Laan)
5d3064b Temporarily add SetNull/IsNull/GetCheapHash to base_uint (Wladimir J. van der Laan)