Add GC_get_prof_stats[_unsafe]() to GC public API

Add GC_get_prof_stats[_unsafe]() to GC public API

* include/gc.h (GC_get_heap_size): Update comment.
* include/gc.h (GC_prof_stats_s): New structure.
* include/gc.h (GC_get_prof_stats): New API function.
* include/gc.h (GC_get_prof_stats_unsafe): New API function (only if
GC_THREADS).
* misc.c (fill_prof_stats): New static function (only if not
GC_GET_HEAP_USAGE_NOT_NEEDED).
* misc.c (GC_get_prof_stats): New API function definition (only if not
GC_GET_HEAP_USAGE_NOT_NEEDED).
* misc.c (GC_get_prof_stats_unsafe): New API function definition (only
if THREADS but not GC_GET_HEAP_USAGE_NOT_NEEDED).