various fixes to IdentityHashSet; now subclasses AbstractSet
various fixes to IdentityHashSet; now subclasses AbstractSet

xph906/SootNew,cfallin/soot,cfallin/soot,plast-lab/soot,cfallin/soot,xph906/SootNew,anddann/soot,plast-lab/soot,mbenz89/soot,anddann/soot,xph906/SootNew,anddann/soot,mbenz89/soot,mbenz89/soot,plast-lab/soot,mbenz89/soot,anddann/soot,xph906/SootNew,cfallin/soot