Add new subdirectory ghc/ for programs that use ghc extensions.  The
old GHC_ONLY directory used to be populated with micro-tests, all of
which appear to have been moved to ghc/tests.

ghc/matrix is Pascal Serrarens' conversion of a Clean benchmark, that
reputedly runs 4 times slower than 0.29 and 40 times slower than the
clean implementation.  We have some ideas why this might be so:
inlining and specialisation isn't working very well for array
operations, so the bounds checking isn't being compiled down into
decent code.  The changes I just made to the bounds representation in
the various array datatypes improve things by 20% or so.