Use pnet_datalink instead of all of pnet

pnet_transport takes a long time to build. It's been especially
painful from within a docker container for reasons I don't care
to understand. pnet_datalink is the only part of pnet we're using
so booting the rest.

src/lib.rs

@@ -59,7 +59,7 @@ extern crate ring;
 extern crate serde;
 #[macro_use]
 extern crate serde_derive;
-extern crate pnet;
+extern crate pnet_datalink;
 extern crate serde_json;
 extern crate sha2;
 extern crate untrusted;