- change to_string() on numbers to to_str()

This makes it more like ls. The --long (-l) argument has been added to get at the old behaviour, and the --links argument is now on -H. I can't crib this behaviour from ls because it shows it by default.

TODO: The terminal size is currently assumed to be 80, and it uses the string length, rather than the width.

And fix bugs found by these tests...

Conflicts:
	unix.rs

First non-trivial change for a while... because this involves reading from the OS, we should cache as much as we can in memory. So, group membership checking is done immediately after reading a group name, as the group structure has already been populated.

Curiously enough, this turned out to be helpful today... Git can't
traverse symlinks, so I had to use hard links instead, and then I had to
work out which files had more than one link at a glance. If I hadn't
already had most of exa written, I'd be stuck using find!