This replaces our existing implementation of PSBTs with a more
straightforward one, adapted from the Bitcoin Core functional tests
framework. This fixes a few flakes that occured because the previous
implementation could produce invalid PSBTs.
The Bitcoin Core implementation is pretty low level and was adapted to
treat mappings as such (the value in the PSBTMap can itself be a
mapping, like for partial signatures or BIP32 derivation paths).
The rest of the diff is adapting the users of PSBT to use the new
implementation and the clearly superior interface (yay!).
