Rework Match <-> i.t.e w/ aggressive flattening. Test transformations.

src/main/scala/leon/purescala/Definitions.scala

src/main/scala/leon/purescala/Extractors.scala

src/main/scala/leon/purescala/ScalaPrinter.scala

src/main/scala/leon/purescala/TreeOps.scala

src/test/resources/regression/transformations/Match.scala

...t/resources/regression/transformations/SimplifyLets.scala

src/test/scala/leon/test/purescala/TransformationTests.scala