Js Txin Ergonomics

[breavyn]

The TxIn constructor requires a script, however when a transaction input is first created, its script is typically not yet known. Therefore providing a script should be optional.

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How I'm currently adding an input

const txin = TxIn.default();
txin.setPrevTxId(prevTxid);
txin.setVOut(prevVOut);
tx.addInput(txin);

How I'd like to add an input

const txin = new TxIn(prevTxid, prevVOut);
tx.addInput(txin);