Make RuleInapplicable a RuleAlternatives with no alternative

src/main/scala/leon/synthesis/Rules.scala

src/main/scala/leon/synthesis/Task.scala

src/main/scala/leon/synthesis/heuristics/ADTInduction.scala

src/main/scala/leon/synthesis/heuristics/IntInduction.scala

...scala/leon/synthesis/heuristics/OptimisticInjection.scala

...n/scala/leon/synthesis/heuristics/SelectiveInlining.scala

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

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

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

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

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

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

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

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

...main/scala/leon/synthesis/rules/IntegerInequalities.scala

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

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