Add Wtinesses class

[SebastienGllmt]

For Yoroi we have a unittest that makes sure that the a transaction got signed by the correct keys. We do this by checking if the witness is correct.

Example usage in Yoroi

const witnesses = signedTx.witnesses();

expect(witnesses.size()).toEqual(1);
expect(witnesses.get(0).to_bech32()).toEqual(
    'witness1q9tuk0s75d4zwahpxzydwr3dfang74s3l0l7whmx8fgjzfz28s9pnpeuhxmvu8ksa3h0hl4guhyusdaa2u93usu0vgggsqlrcswt7cq9vlq8pr'
);