Now it can refuse to resize when it would disturb the metadata tracking
strategy, resulting in smaller code size, a simpler implementation, and
less fragmentation.
The previous version had a fatal flaw: it did ensureCapacity(1) on the
freelist when allocating, but I neglected to consider that you could
free() twice in a row. Silly!
This strategy allocates an intrusive freelist node with every
allocation, big or small. It also does not have the problems with resize
because in this case we can push the upper areas of freed stuff into the
corresponding freelist.
