Addding elementary support for sets in Orb

src/main/scala/leon/invariant/structure/Constraint.scala

...ala/leon/invariant/templateSolvers/NLTemplateSolver.scala

...scala/leon/invariant/templateSolvers/TemplateSolver.scala

...ain/scala/leon/invariant/util/ExpressionTransformer.scala

...egression/verification/purescala/invalid/PreInSpecs.scala

.../regression/verification/purescala/valid/PreInSpecs.scala

...y-datastructures/ManualnOutdated/LazyMegeSort1-Time.scala

...cases/lazy-datastructures/ManualnOutdated/TestArray.scala

...es/lazy-datastructures/ManualnOutdated/TestGenerics.scala

testcases/lazy-datastructures/ManualnOutdated/TestSets.scala

...ctures/ManualnOutdated/WeightedScheduling-strategy2.scala