graphs: moving to basis.
sets: adding adjoin-all.
compiler.units: changing definitions-changed to use a set.
compiler.units: simpler update-existing? check.
compiler.units: simplify checks.
compiler.tree.tuple-unboxing: simpler checks.
tools.scaffold: more use of hash-sets.
compiler.cfg.ssa.construction: more use of hash-sets.
compiler.cfg.loop-detection: more use of hash-sets.