add SCM_HEAP_OBJECT_BASE

add SCM_HEAP_OBJECT_BASE

* libguile/tags.h (SCM_HEAP_OBJECT_BASE): New macro.  Given a SCM,
  returns a pointer to the start of its memory area on the heap.

* libguile/bytevectors.c:
* libguile/fluids.c:
* libguile/foreign.c:
* libguile/gc.h:
* libguile/guardians.c:
* libguile/numbers.h:
* libguile/ports.c:
* libguile/smob.c:
* libguile/struct.c:
* libguile/weak-set.c:
* libguile/weak-table.c:
* libguile/weak-vector.c: Use it.