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.
