Proposed fix to show_usage to support free list implementations that allocate all HeapObjects instead of all-but-one.

Changed show_usage in low-level.js to start from 0 free elements instead of 1, and to check ptr != null instead of ptr != freelist tail. In essence, exchanged the assumption that for ptr.deref()._next != freelist tail, _next != null, for the new assumption that if free_list_tail is non-null that the HeapObject it points to has _next === null.