Thwart compiler optimizations.
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.
Purge/zero sub-chunk huge allocations as necessary.

Purge trailing pages during shrinking huge reallocation when resulting
size is not a multiple of the chunk size.  Similarly, zero pages if
necessary during growing huge reallocation when the resulting size is
not a multiple of the chunk size.