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!

This is behaviour cribbed from ls. Even though I've never personally used it, I'm sure someone has a use for it.

This requires the dir field to store a reference to the path that
created it, so that path can be used to join onto the filename so it can
be properly statted.

There doesn't seem to be an io::FileType entry for character special
devices or sockets, so these fall under the io::UnknownType entry. Which
also gets highlighted in yellow, for precisely that reason.