[ecashaddrjs] Stop using buffer and webpack

Summary:
We should not need webpack to build a module. This is an unnecessary complication. It is being used to polyfill Buffer, which does not work in web.

Stop using Buffer. In this way we can get rid of the webpack complication.

(Discovered while looking at what had to be done for eslint and arc lint to work on all libs)

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17170