Remove dependencies on NaiveDataGen, fix testgen when no input

src/main/scala/leon/datagen/GrammarDataGen.scala

src/main/scala/leon/datagen/NaiveDataGen.scala

src/main/scala/leon/datagen/VanuatooDataGen.scala

src/main/scala/leon/purescala/Constructors.scala

src/test/scala/leon/test/purescala/DataGen.scala