Implement public interface for GC statistics.
We add a new RTS flag -T for collecting statistics but not giving any new inputs. There is one new struct in rts/storage/GC.h: GCStats. We add two new global counters current_residency and current_slop, which are useful for in-program GC statistics. See GHC.Stats in base for a Haskell interface to this functionality. Signed-off-by: Edward Z. Yang <email@example.com>