PSET-v2 Output Serialisation

If i understood correctly keypairs should be serialised in order as defined by [BIP 174](https://github.com/bitcoin/bips/blob/master/bip-0174.mediawiki) and in [here].(https://github.com/ElementsProject/elements/blob/master/doc/pset.mediawiki)

Currently this is not the case for output serialisation as script is serialised before amount in [here](https://github.com/ElementsProject/elements/blob/43665e3ca0229219fff2e7f0a3da1d7257855bda/src/psbt.h#L998).
As per BIP 174 it should be opposite.

I can see @achow101, in PSBT2 branch, follows order defined in BIP 174/370, check [here](https://github.com/achow101/bitcoin/blob/59d0af20e4fa46a059af30be286d09c075e0331f/src/psbt.h#L582).