ed25519: support signing structs w/ bcs canonical serialization

+ Add `Signable` trait, which mandates signed structs use a domain
  separation value.

+ Add `Signed` type, which wraps a recently signed/signature verified
  struct and is only producable inside the `ed25519` module. I'm hoping
  this can help with API design, so functions can require "hey you must
  have signature verified this struct or else I won't accept it".

+ Use `bcs` for binary canonical serialization since I already trust it.
  Could use something else in the future, but this works for now.

Author: phlip9

Cargo.lock

@@ -43,6 +43,8 @@ anyhow = "1"
 argh = "0.1"
 # async fn's in trait methods
 async-trait = "0.1"
+# Binary canonical serialization format
+bcs = "0.1"
 # Working with bytes
 bytes = "1"
 # Conveniently write #[cfg(..)] if-else statements without repeating clauses