Merge #12752: [MOVEONLY] Move compressor utility functions out of class

Summary:
76a9aac Move compressor utility functions out of class (Pieter Wuille)

Pull request description:

This is a refactor from #10785 with no functionality change.

Move the compresion utility functions out of CScriptCompressor, as a preparation for making the class templated. I'm submitting it as a separate PR as I think it's a general improvement to code readability, and to reduce the diff further on.

Tree-SHA512: 3b3d17c2b96e43f752f512dd573296a6bb15cae165fbe3c79212a0970f5196a62a59a821d5100f29638af1e7461c9171f3dccb8112f005ee08da0ec7fe0073fd

Backport of Core PR12752
https://github.com/bitcoin/bitcoin/pull/12752/

Test Plan:

make check

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

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3903