Handle the somewhat half-mapped alias TransactionOutputs

Handle the somewhat half-mapped alias TransactionOutputs

TransactionOutputs is actually a
C2Tuple_TxidCVec_C2Tuple_u32TxOutZZZ, but with a type alias that
gives it a more readable name in a few places. Sadly, we don't
support type aliases, so we have to just map it back before building
our bindings.