PSETs have a bit different requirements. The witness_utxo needs
the asset tag + values, and these should also be added to the PSET
struct separately as well. To do this, we create a new 'init' method for
elements inputs, which takes care of the elements specific things.
will either use a temporary psbt (and not munge the passed in psbt)
or will finalize in place -- finalization erases most of the signature
metadata from the psbt struct
calling `wally_psbt_finalize` doesn't return a status indicator; instead
you must call `psbt_is_finalized` to check that it's eligible for
'extraction' -- extraction will fail if the psbt is not in a finalized
state.
Move all psbt creation into single method, new_psbt
note that if a psbt is init'd for a transaction that's
deserialized with scripts etc already attached, then set_global_tx
will fail. instead, we empty all of this out first.
if the tx is being re-init'd from a tx source that had a psbt attached
(e.g. fromwire_) then the script/witness data will get populated
appropriatel from there.
niftynei