Remove potential signature malleability over issuance inputs

[instagibbs]

Serialization leaves the exact inputs ambiguous, mixing single byte 0 and full assetIssuance data in the signature hash: https://github.com/ElementsProject/elements/blob/master/src/script/interpreter.cpp#L1585

This could allow malleability where signatures are replayed on other transactions, though it seems most are prevented by sighash_all and surjection proofs.

A future sighash version could fix this by canonically serializing all the inputs with varints.

[apoelstra]

Fixed by #1002