Support for lowR in BitcoinJsWalletProvider

[matthewjablack]

bitcoinjs-lib added the ability to always have low R values during transaction creation: bitcoinjs/bitcoinjs-lib#1442

This means signature size is at most 71 bytes, and maxWitnessLen goes from 108 to 107

It also has other benefits, such as saving several bytes on txs for multisigs: bitcoinjs/bitcoinjs-lib#1175 (comment)

Would be great if there were a way to force low R values during transaction creation, or default to this for CAL.

[matthewjablack]

This could be as simple as modifying the lowR value in ECPair class when returning keyPair from BitcoinJsWalletProvider: https://github.com/bitcoinjs/bitcoinjs-lib/blob/533d6c2e6d0aa4111f7948b1c12003cf6ef83137/src/ecpair.js#L19

[kraikov]

@monokh any actions here?