[ecash-script] Add util function swapEndianness

Summary:
Add new function swapEndianness

Use this in consumeNextPush in place of manual implementation. Export function for developer use.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14002