Modify uint256's comparison to use proper endianess

Summary: This makes the comparison consistent with arith_uint256 .

Test Plan: Updated unit tests.

Reviewers: #bitcoin_abc, schancel, jasonbcox

Reviewed By: #bitcoin_abc, schancel, jasonbcox

Subscribers: jasonbcox, schancel, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1527