Replace mod redjubjub by pub use redjubjub_zebra as redjubjub

[hdevalence]

We needed a RedJubjub implementation for use in Zebra (cf ZcashFoundation/zebra#125), so I made a small, mostly self-contained one for that purpose using the new version of the jubjub crate. It's not yet on crates.io just because we didn't publish it yet; it's otherwise functionally complete. One nice thing about its API is that unlike the existing redjubjub implementation, it enforces the BindingSig/SpendAuthSig distinction at the type level.

I think that it would integrate nicely into zcash_primitives, replacing mod redjubjub; with a re-export pub use redjubjub_zebra as redjubjub;, and I would be happy to prepare such a PR, but because it requires propagating type changes through zcash_primitives, I wanted to check first rather than doing work that doesn't fit with future plans for librustzcash.

This would supersede #179.

[str4d]

Eventually yes, this seems reasonable. However, zcash_primitives et al don't yet use the jubjub crate (it's the next step in the refactor), and I suspect it will be more complicated to try and pull this in beforehand. You're welcome to try though - it might be that the wrappers around redjubjub to map from jubjub to zcash_primitives::jubjub aren't too terrible.

Additionally, we'll be vendoring jubjub inside our codebase while doing the refactor, so that we can adapt jubjub's API as necessary while we go (in particular to end up with jubjub implementing the ff and group traits, which themselves will be adapted to more closely match jubjub's API), which probably means vendoring redjubjub as well.

[str4d]

Instead of a re-export, we'll just drop the redjubjub module entirely (#581).