Refactor the new CombinePSBT, FinalizePSBT, and FinalizeAndExtractPSBT
general-purpose functions out of the combinepsbt and finalizepsbt RPCs,
for use in the GUI code.
Remove the op== on PartiallySignedTransaction, which only checks that the
CTransactions are equal. Instead, check this directly in Merge, and return
false if the CTransactions are not equal (so the PSBTs cannot be merged.)
Move non-wallet PSBT code to src/psbt.{h,cpp}, and PSBT wallet code to
src/wallet/psbtwallet.{h,cpp}. This commit contains only code movement (and
adjustments to includes and Makefile.am.)
