Introduce GrammarDataGen for using faster evaluators

src/main/scala/leon/codegen/CodeGenParams.scala

src/main/scala/leon/codegen/CompilationUnit.scala

src/main/scala/leon/codegen/runtime/ChooseEntryPoint.scala

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

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

src/main/scala/leon/evaluators/CodeGenEvaluator.scala

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

src/main/scala/leon/purescala/ExprOps.scala

src/main/scala/leon/repair/Repairman.scala

src/main/scala/leon/synthesis/ExamplesFinder.scala

src/main/scala/leon/synthesis/SynthesisPhase.scala

src/main/scala/leon/synthesis/SynthesisSettings.scala

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

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

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

src/main/scala/leon/synthesis/utils/ExpressionGrammar.scala