[ecash-lib] Add TxBuilder to build Bitcoin transactions

Summary: Allows building Bitcoin transactions, able to handle any crazy covenant; able to calculate leftover outputs precisely.

Test Plan: npm run test && npm run build && npm run integration-tests

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D15963