Split utility files into separate library.

Everything in src/common has been moved to src/uucore. This is defined
as a Cargo library, instead of directly included. This gives us
flexibility to make the library an external crate in the future.

Fixes #717.

tests/hashsum.rs

@@ -1,10 +1,6 @@
 #[macro_use]
 mod common;
 
-
-
-
-
 macro_rules! get_hash(
     ($str:expr) => (
         $str.split(' ').collect::<Vec<&str>>()[0]