[ecash-lib] Add parseSlp
Summary:
Parses an SLP OP_RETURN Script.
See doc comment of parseSlp for details.
Test Plan: npm test
Reviewers: Fabien, #bitcoin_abc, bytesofman
Reviewed By: Fabien, #bitcoin_abc, bytesofman
Subscribers: bytesofman
Differential Revision: https://reviews.bitcoinabc.org/D17871