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 <ezyang@mit.edu>