Add support for CAT

Summary:
Add handling for CAT opcode when the monolith script flag is enabled.

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

Test Plan: make check

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

Reviewed By: #bitcoin_abc, O1 Bitcoin ABC, jasonbcox

Subscribers: jasonbcox, shadders, danconnolly, teamcity, macros

Maniphest Tasks: T307

Differential Revision: https://reviews.bitcoinabc.org/D1227