PSET: Put PSBT_OUT_AMOUNT back in the same position as in PSBT
PSET: Put PSBT_OUT_AMOUNT back in the same position as in PSBT

While the order of fields is not explicit in the PSBT/PSET specifications,
third parties need to be able to support both with a single implementation.
Keeping the PSBT fields in the same order makes this significantly
easier.

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