Rework the semantics of rules, their applications/instantiation, results

src/main/scala/leon/synthesis/rules/OptimisticGround.scala

...main/scala/leon/synthesis/rules/UnconstrainedOutput.scala

src/main/scala/leon/synthesis/rules/Unification.scala

src/main/scala/leon/synthesis/rules/UnusedInput.scala

src/main/scala/leon/synthesis/search/AndOrGraph.scala

...ala/leon/synthesis/search/AndOrGraphPartialSolution.scala