Add two new macros and refactor the `combine` functions to use them.

Refactor only, no logic changes.

It does not make sense to allow our `v0` implementation to combine two
PSBTs with different versions, error if this is attempted.

While we are at it, move the `CombineError` into `global` because that
is where it originates. Although this is not important in `v0`, over in
`v2` there will be different `CombineError`s for the different maps -
lets be uniform already.

Add a function `combine` to the `v0` module as we do in `v2`. While we
are at it make all the docs and calling conventions uniform.

If an attempt is made to combine two PSBTs with different values for the
global map fields `version` or `tx_version` return an error.

This is in line with what we did in `v0`.

The current version of `Input::combine` is copied from the `v0` module
and is not correct for `v2` - fix it.

As we did in other places use plural form for fields in the `Output`
map.

As we did for `v2::Input`.

The current version of `Output::combine` is copied from the `v0` module
and is not correct for `v2` - fix it.

We have now fully replaced the usage of this private macro, remove it.