cleaner (and more correct) separation of array operation

src/main/scala/leon/ArrayTransformation.scala

src/main/scala/leon/Evaluator.scala

src/main/scala/leon/FairZ3Solver.scala

src/main/scala/leon/purescala/PrettyPrinter.scala

src/main/scala/leon/purescala/Trees.scala

testcases/regression/valid/Array3.scala