RedJubJub signatures

[hdevalence]

In addition to #109, we also need a library for RedJubJub signatures. One exists in librustzcash/zcash_primitives but it is coupled to the rest of that library, and the zkcrypto/jubjub implementation will soon be using bls12_381 directly.

[hdevalence]

The library now exists: https://github.com/ZcashFoundation/redjubjub-zebra ; I could either a) update #105 to include it or b) do the update in a subsequent PR. I have a slight preference towards a) if that's OK, since there are other refinements needed for #123 beyond just the Redjubjub changes.

[dconnolly]

The library now exists: https://github.com/ZcashFoundation/redjubjub-zebra ; I could either a) update #105 to include it or b) do the update in a subsequent PR. I have a slight preference towards a) if that's OK, since there are other refinements needed for #123 beyond just the Redjubjub changes.

a) sounds good to me!