[ecash-lib] Export Tx.toHex()

Summary:
This is just a convenience function that makes it possible to get the serialized hex of a tx using ecash-wallet without explicitely requiring the toHex function from ecash-lib.

Depends on D18394.

Test Plan:

npm test

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D18396