Add support for BIN2NUM opcode

Summary: As per title. As the whole number system is LE, I went went for LE encoding. This doesn't seems like a great idea to fubar the scrpipt system even more than it already is by introducing mixed endianess in it.

Test Plan: Added various test cases.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, schancel, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1220