Faster IntSet by using BitMaps in the lower branches

[Henning Thielemann]

On Sat, 17 Sep 2011, Joachim Breitner wrote:

> for one of my applications, I am generating huge IntSets (and IntMaps),
> and I am regularly hitting the memory constraints of my machine. Hence I
> am looking for a better implementation of IntSets with smaller memory
> foot print, at least in the case of dense sets, e.g. those where it is
> likely that a few values are equal in all but the lower 5 or 6 bits.

It sounds reasonable for me. I guess in my cases the tree will often only 
contain a few bit-vector nodes.