Restore functionality of CEGIS, more cleanup

src/main/scala/leon/grammars/ExpressionGrammar.scala

src/main/scala/leon/grammars/Grammars.scala

src/main/scala/leon/grammars/SimilarTo.scala

src/main/scala/leon/grammars/aspects/SimilarTo.scala

src/main/scala/leon/grammars/aspects/TypeDepthBound.scala

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

src/main/scala/leon/purescala/GenTreeOps.scala

src/main/scala/leon/purescala/TypeOps.scala

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

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

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

src/main/scala/leon/synthesis/rules/unused/TEGLESS.scala

...test/scala/leon/integration/grammars/SimilarToSuite.scala