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.
There are several areas needing improvement:
1) add tests for hard links
2) add implementation for uncommon flags (-d, -L, -n, -P, -r)
3) align error messages more closely with GNU implementation
