Fix Search, fix problem cost, re-introduce ADT Split

src/main/scala/leon/synthesis/Cost.scala

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

src/main/scala/leon/synthesis/Synthesizer.scala

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

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

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