Add support for enumerating the reachable objects in the heap

Add support for enumerating the reachable objects in the heap
(Apply part of commit dbf2db8 from 'paulbone/mercury7_4_pbone' branch.)

This can be used to do heap profiling, helping the developer work out which
objects are live or what kinds of objects account for a lot of their heap
space.

* alloc.c:
* include/gc.h:
* include/private/gc_priv.h:
* reclaim.c:
    As above.