After #1896 is merged, I think we should remove ProvenTransactionBuilder and simply add a ProvenTransaction::new constructor. I don't see the reason why the builder code is necessary, and removing it would simplify the code a bit and maybe deduplicate some docs. The new constructor should do the validation that ProvenTransaction::validate does, instead of allowing construction of invalid types. In particular, we need to make sure that the proven transaction is validated during deserialization.