Write FFI bindings and function pointers for ed25519-dalek

As part of our efforts to get a few modules in Tor written in Rust for 0.3.3, an exceptionally easy candidate is our ed25519 code, given that the current code is already highly modularised, taking function pointers to implement an interface. I wrote ed25519-dalek, and I recently revised the API to be a very close match to what tor expects, so I believe this task should be extremely easy, and a prime candidate for someone newer to Rust who wishes to learn about writing FFI. (I'm happy to pair program on this too! Also on anything else, but this too.)