heap headers

[Henry Cejtin]

One  of  the  tricks  you  can  do  in  a  generational collector is to store
different types of things in different places (in older generations)  and  so
not  have  a  tag  on  every item describing it.  This only matters for small
things where the  tag  represents  a  large  percentage  of  the  size.   The
canonical example is list pairs.  One notion would be to make things that are
12 bytes or less go into different regions, divided according to the size and
pointer/non-pointer aspects.  It isn't worth it for the nursery, but probably
is for everything else.

Just a thought.