refactor PSBT file

[jasonandjay]

The motivation for doing this is to simplify the PSBT code and put together the more closely related logic through the split mode to facilitate subsequent maintenance and splitting.
Mentioned here before #1993
So I did three things

1. Move payments-related logic from psbtutil to payments, which is more appropriate.
2. Split the auxiliary functions in class PSBT into global, input and output in the PSBT directory
3. I would like to hear your opinions. Do you have any intention to dismantle it separately? Like BIP174, I can do this if I have the opportunity.