Additional headers under include/experimental/

[Jonathan Wakely]

I'm working on parts of the Filesystem TS and think it makes sense to
split it up into separate headers for path, directory utils and the
free functions, then include those from <experimental/filesystem>,
otherwise it's going to be a whopper.

Currently I've created include/experimental/fs_path.h for the path
class, does that seem OK?

Would a new include/experimental/bits directory be better, so that
only official headers required by a TS are in include/experimental?
(That would imply moving string_view.tcc to another directory).