Exhaustive parsing checks

Right now, we only sanity-check the PSBTs in from_psbt_serialized() are sane with regard to our internal assumptions (unwrap()s basically). This may be enough, but we may also want complete parsing checks to be sure that whether a transaction is parsed or created with the constructor it will hold to our requirements (eg, no dust amounts, sane feerate, etc..). This is quite some more hairy code though..