Accommodate the heap size to the working set size

The GC can have a lot of pressure if a collection recovers very little
memory, but enough to fulfill the current memory request, causing a lot of
collections with too little gain (when the key to a efficient GC is
recover the bigger amount of memory with as little work as possible).

This effect is greatly reduced when using eager allocation, because
eventually the GC will allocate more memory, but heap minimization can
trigger this effect again.

This patch adds an option, min_free, which specifies the minimum
percentage of heap that should be free after a collection. If the free
heap is less than min_free% of the total heap, the GC will allocate a new
pool, big enough to fulfill that requirement. If the free heap is bigger
than min_free% of the total heap, the GC will try to release some pools to
the OS to keep the heap occupancy near min_free%.