Don't collapse singleton BitmapIndexed nodes in filter
Don't collapse singleton BitmapIndexed nodes in filter

This (incorrect) attempt at an optimization could cause a
BitmapIndexed node to move to another level, where its bitmap wasn't
valid.  Closes #33

hvr/unordered-containers,bgamari/unordered-containers,rimmington/unordered-containers,ivan-m/unordered-containers