[ecash-herald] Implement ecash-script for OP_RETURN parsing

Summary:
T2972

Simplify existing parseOpReturn function to use ecash-script library.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14003