gc: free heaps that become empty.

On glibc, our heap allocation requests are considered
large and handled via mmap; when we free a heap, the memory
is returned to the OS via munmap.

* gc.c (sweep): If every object in a heap is freed,
we free the entire heap, taking care to also reset the free
list to the state before those objects were added to it.
The free list may still contain objects from that same heap
that were not just added to it (they were freed in a previous
GC pass), so we must walk the free list to find the remaining
objects and remove them. The Valgrind debugging logic (opening
access and closing while walking the list) was too cumbersome
so it's done in two passes: open access to the whole free
list, process it, close off what is left.

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