[ecash-lib] Add a bunch of helpers for Script

Summary:
Add helper functions fromOps, ops, cutOutCodesep, isP2sh, p2sh, p2pkh, p2pkhSpend to Script.

Add ScriptOpIter, which is returned by Script.ops to iterate over the Ops of a Script.

Test Plan: npm run test && npm run build

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D15952