Split GC.c, and move storage manager into sm/ directory
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.