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
When creating tests, we could pass a String, &str, or [u8] to the
standard input. Previously, all arguments had to be manually converted
to a reference to a [u8]. This update makes it more ergonomic.
