`struct tx_parts` is just a txid and a bunch of inputs and outputs,
some of which may be NULL.
This is both a nod towards a future where we (or our peer) can combine
HTLCs or (in an eltoo world) commitments, although for the moment all
our tx_parts will be complete.
It also matches our plan to split `bitcoin_tx` into two types: this
`struct tx_parts` where we don't know input amounts etc, and `psbt`
where we do.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
