fuse.hs: implement rmdir
fuse.hs: implement rmdir

LFS smallfile sort-of runs now, except there seem to be two bugs
in the Haskell code somewhere:

(1) occasionally i get this crash when running smallfile:

    fuse: schedule: re-entered unsafely.
      Perhaps a 'foreign import unsafe' should be 'safe'?

(2) starting ./fuse without a disk.img (which should create it) produces:

    Initializing file system
    Segmentation fault (core dumped)

mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl