Recently we (*cough* Tobin) made the `Map` trait private and neglected
to add a public API for combining together two PSBTs. Doing so broke the
`psbt` module.
Pull the merge logic out of the `Map` trait and put it in methods on
each individual type (`Input`, `Output`, `PartiallySignedTransaction`).
Doing so allows for simplification of return types since combining
inputs/outputs never errors.
Use the term 'combine' instead of 'merge' since that is the term used in
BIP 174.
Tobin Harding