libwally has a quirk where the finalize method will fail to 'completely'
finalize an input's parts if either the final_scriptsig or
final_redeemscript fields are set
since we manually set the final_witness stack here, we also need to
fully finalize the redeemscript -> final_scriptsig here as well.
There's a few structs/wire calls that only exist under experimental features.
These were in a common file that was shared/used a bunch of places but
this causes problems. Here we move one of the problematic methods back
into `openingd`, as it's only used locally and then isolate the
references to the `witness_stack` in a new `common/psbt_internal` file.
This lets us remove the iff EXP_FEATURES inclusion switches in most of
the Makefiles.
niftynei