Added OP_REVERSEBYTES+implementation, added (always disabled) activation flag

Summary:
Implementation of OP_REVERSEBYTES is very straightforward. T707.

Previously named OP_REVERSE, then OP_BSWAP, then OP_ENDIAN_REVERSE.

Test Plan: Tests listed in the OP_REVERSEBYTES PR: https://github.com/EyeOfPython/bitcoincash.org/blob/master/spec/op_reversebytes.md

Reviewers: deadalnix, markblundeberg, Fabien, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, markblundeberg, O1 Bitcoin ABC, #bitcoin_abc

Subscribers: jasonbcox, Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D4871