Normalize size classes.

Normalize size classes to use the same number of size classes per size
doubling (currently hard coded to 4), across the intire range of size
classes.  Small size classes already used this spacing, but in order to
support this change, additional small size classes now fill [4 KiB .. 16
KiB).  Large size classes range from [16 KiB .. 4 MiB).  Huge size
classes now support non-multiples of the chunk size in order to fill (4
MiB .. 16 MiB).