Add basic structure of `cp`
Implement basic file copying

Assumes there is only one source and one destination. Corresponds to the `cp
SOURCE DEST` usage documented in `man cp`. No options are implemented.
Remove file existence check; add equivalency check

GNU `cp` does not require that the destination not exist in the case of `cp
SOURCE DEST` usage as I previously thought; that check has been removed.
However, it does error if the paths given refer to the same file, (traversing
symlinks); a check for that was added.
Refactor equivalence test into function
Add util.rs

Apparently I missed this when I factored out the equivalency check
link -> crate_id; link is deprecated
Fix misc errors

Honestly I'm utterly baffled as to how this compiled
Move option parsing into main
Add copyright header to util.rs