In preparation for parallel GC, split up the monolithic GC.c file into
smaller parts.  Also in this patch (and difficult to separate,
unfortunatley):
  
  - Don't include Stable.h in Rts.h, instead just include it where
    necessary.
  
  - consistently use STATIC_INLINE in source files, and INLINE_HEADER
    in header files.  STATIC_INLINE is now turned off when DEBUG is on,
    to make debugging easier.
  
  - The GC no longer takes the get_roots function as an argument.
    We weren't making use of this generalisation.