Simplify/clean up extractors

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

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

src/main/scala/leon/evaluators/RecursiveEvaluator.scala

src/main/scala/leon/frontends/scalac/CodeExtraction.scala

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

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

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

src/main/scala/leon/solvers/smtlib/SMTLIBCVC4Solver.scala

src/main/scala/leon/solvers/smtlib/SMTLIBSolver.scala

src/main/scala/leon/solvers/z3/AbstractZ3Solver.scala

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

...es/regression/verification/purescala/invalid/Array4.scala