Add current benchmarks, new benchmark List[T].indexOfOpt with variants

testcases/synthesis/PrimeHeuristic.scala.off

-import leon.lang._
-
-object PrimeHeuristic {
-  def maybePrime(n: Int): Boolean = n match {
-      case 2 * k     => false
-      case 3 * k     => false
-      case 6 * k - 1 => true
-      case 6 * k + 1 => true
-  }
-
-}