Upgrade to rand_core 0.6

[hdevalence]

There's a new version of the rand_core traits, and it would be great for ecosystem users outside of Zebra to be able to use it. This requires a version of the underlying curve library with support for rand_core 0.6, which is provided by curve25519-dalek-ng 4.0, as in #38.

However, this requires a major version bump, which poses a slight obstacle for the legacy zcash / zip215 split that the crate currently handles by using the 1.x series for the legacy zcash validation rules and the 2.x series for the zip215 rules. This versioning scheme means that there's not a clean way to make breaking changes to the API for legacy zcash validation rules -- at the time, I was expecting that there wouldn't be a need to do that prior to the full deprecation of the legacy rules in the Canopy network upgrade, at which point there's no need to maintain that code.

My sense is that the cleanest thing to do would be to only apply the rand_core bump on the 2.x zip215 series to get a 3.x version, since the primary benefit is for ecosystem users outside of Zebra. The only applications that need the 1.x series with the legacy rules are Zebra and zcashd, and neither have a need to upgrade to the latest rand_core prior to the Canopy upgrade.