Add small run utilization to stats output.

Add the 'util' column, which reports the proportion of available regions
that are currently in use for each small size class.  Small run
utilization is the complement of external fragmentation.  For example,
utilization of 0.75 indicates that 25% of small run memory is consumed
by external fragmentation, in other (more obtuse) words, 33% external
fragmentation overhead.

This resolves #27.