BIP352: PSBT support

[andrewtoth]

@achow101

Inputs can be added and removed if Inputs Modifiable is set, so I don’t think it’s a good idea to have anything that relies on the ordering of inputs to be consistent. Having any field that includes a list of indexes could become messed up by an unaware constructor that adds an input in the wrong place.

If we can only add silent payment outputs if Inputs Modifiable is not set, then this point is moot, no?

I don’t think we should dismiss this optimization. Consider the common case of a wallet with 10 small utxos, and it wants to make a single payment with all of them to a silent payment address. Without consolidating all shares and proofs, the hardware wallet signer will need to compute 10 times more shares and proofs.