The `SpendAuth` test vectors were generated using
`FixedGenerators::SpendingKeyGenerator`, and the `Binding` test vectors
were generated using `FixedGenerators::ValueCommitmentRandomness`.
This ran into problems with Clone/Copy bounds -- it seems like the
derived impls require that the phantom type T also be Clone / Copy /
Debug for the type to be. This commit does a hacky fix that makes it
work for now, but it should be cleaned up later.