Tracking issue - PSBT refactoring & PSBTv2

[tcharding]

This is a tracking issue for ongoing PSBT work, including but not limited to implementing support for PSBT v2 BIP-370.

For reference we do support PSBT v0 BIP-174.

There is a GitHub project already for this epic: https://github.com/rust-bitcoin/rust-bitcoin/projects/1

There is not currently a champion for this epic: #787

[tcharding]

Field names discussion: #1054

[Kixunil]

I think one of the first, significant steps is to put TxIn into Input so that consistency is guaranteed.For consideration CheckedPsbt which additionally guarantees that you can get a correct previous output for each input.

[tcharding]

Closing in favour of #1115