[Chronik] Add Script::iter_ops and ScriptOpIter

Summary:
Iterates over the operations in a Script. This can either be a single opcode, or an opcode which pushes some data following it in the bytecode.

This is important for parsing scripts, e.g. for SLP.

Test Plan: ninja check-crates

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13791