Implement a Ed25519 batch verify, use it in the RootHash

[Yawning]

With Ed25519 it's possible to do batch verification which optimizes verifying a set of signatures vs a set of public keys all at once. As our committee sizes grow (and we need to start validating SGX commits), it's probably worth thinking about utilizing this technique to reduce the time spent validating commits.

golang.org/x/crypto/ed25519 last I checked is still agl's Go port of djb's ref10, but he left out the batch verification implementation. Porting the rest of it over should be a straight forward drop-in process.

If signature verification/signing ends up being a perf bottleneck, it's possible to implement all of these much faster than, ref10 as well, though doing it without massive pain will require Go 1.12 (for math/bits integer intrinsics).

[Yawning]

https://github.com/oasislabs/ed25519 provides this, once we are confident with the code, we could switch all of our Ed25519/X25519 over to our package.