Prepare for re-enabled opcodes for the May 2018 protocol upgrade

Summary:

• Rename OP_SUBSTR to OP_SPLIT
• Add OP_BIN2NUM & OP_NUM2BIN, replacing OP_LEFT and OP_RIGHT
• tests in script_tests.json were updated to use the renamed opcodes
• Also added FIRST_UNDEFINED_OP_VALUE, the first value of undefined opcodes - OP_NOP10+1 was being used for this purpose which is confusing

Co-authored-by: Joshua Yabut <yabut.joshua@gmail.com>
Co-authored-by: Marcos Mayorga <mm@mm-studios.com>
Co-authored-by: Daniel Connolly <daniel@dconnolly.com>

Test Plan:
make check
test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, O1 Bitcoin ABC, schancel, movrcx

Reviewed By: #bitcoin_abc, O1 Bitcoin ABC, schancel

Subscribers: schancel, teamcity, danconnolly, deadalnix

Maniphest Tasks: T303

Differential Revision: https://reviews.bitcoinabc.org/D1141