Move json into it's own crate

Not everyone needs json, so no reason to force people to build
it.

serde_tests/benches/bench.rs

@@ -3,6 +3,7 @@
 extern crate num;
 extern crate rustc_serialize;
 extern crate serde;
+extern crate serde_json;
 extern crate test;
 
 include!(concat!(env!("OUT_DIR"), "/bench.rs"));