refactor: Make CScriptVisitor stateless

Summary:

CScriptVisitor was added in core#1357 and the visitor return type was never used. Now CScriptVisitor is stateless and CScript is the return type.

This is a backport of core#18863

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9939