This closes a gap in the API where it was impossible to retry items in a failed
batch, because the opaque Item type could not be verified individually.
* Pulls in some traits and methods from curve25519-dalek around the
vartime multiscalar multiplication.
* Move scalar mul things we want to upstream to jubjub to their own crate
* Make Verify agnostic to the SigType
Co-authored-by: Henry de Valence <hdevalence@hdevalence.ca>
Co-authored-by: Jane Lusby <jlusby42@gmail.com>
The `SpendAuth` test vectors were generated using
`FixedGenerators::SpendingKeyGenerator`, and the `Binding` test vectors
were generated using `FixedGenerators::ValueCommitmentRandomness`.