Add setter and getter for GC_on_heap_resize;

Add setter and getter for GC_on_heap_resize;
change GC_on_heap_resize argument type from size_t to GC_word

* include/gc.h (GC_on_heap_resize_proc): Declare new type.
* alloc.c (GC_on_heap_resize): Change type to GC_on_heap_resize_proc
(which uses GC_word argument instead of size_t and GC_CALLBACK calling
conventions).
* include/gc.h (GC_on_heap_resize): Likewise.
* alloc.c (GC_expand_hp_inner): Do not cast GC_heapsize value in
GC_on_heap_resize call.
* include/gc.h (GC_on_heap_resize): Refine comment.
* include/gc.h (GC_set_on_heap_resize, GC_get_on_heap_resize): New
API functions.
* misc.c (GC_set_on_heap_resize, GC_get_on_heap_resize): Likewise.