Extend the event log with a Claim event to claim possession

Unlike a Discovery event, a Claim event associates a public key
with a hash. It's intended to to be used to claim ownership of
some hashable data. For example, a graphic designer could claim
copyright by hashing some image they created, signing it with
their private key, and publishing the hash-signature pair via
the historian. If someone else tries to claim it as their own,
the designer can point to the historian's log as cryptographically
secure evidence that the designer's copy existed before anyone
else's.

Note there's nothing here that verifies the first claim is the actual
content owner, only that the first claim almost certainly happened
before a second.

src/lib.rs

@@ -3,7 +3,9 @@ pub mod log;
 pub mod historian;
 extern crate generic_array;
 extern crate rayon;
+extern crate ring;
 extern crate serde;
 #[macro_use]
 extern crate serde_derive;
 extern crate sha2;
+extern crate untrusted;