Refactor MinimalizeBigEndianArray t be part of CScriptNum

Summary: Also make the logic LE, like everything related to numbers

Test Plan: Move test case over to scriptnum_tests.cpp . Add check against CScriptNum::IsMinimallyEncoded to make sure both are consistent.

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1219