`Directory::path()` returning `ErrorOr` makes it awkward to use, and all
current users create a Directory with a path. If we find we need
pathless directories later, we can come up with a clever solution
then. :^)
We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
Since we already have the directory open, let's have an API to fchown()
the underlying file descriptor instead of forcing clients to do another
path lookup.
Core::Directory represents an existing directory on the system, and it
holds an actual file descriptor so that the user can be sure the
directory stays in existence.
