[ecash-lib] Add parseAlp

Summary:
Parses an ALP pushdata from eMPP.

See doc comment of parseAlp for details. Parallels D17871.

Also add readU48 to Bytes, so we have nice error messages.

Test Plan: npm test

Reviewers: Fabien, bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D17900