Use sha256 hashes instead of Rust's builtin hasher.

Causes a 20x performance degradation. Enabling asm did not
speed things up.

src/lib.rs

@@ -1,5 +1,7 @@
 #![cfg_attr(feature = "unstable", feature(test))]
 pub mod log;
 pub mod historian;
+extern crate digest;
 extern crate itertools;
 extern crate rayon;
+extern crate sha2;