Label -> NonTerminal. Remove view bounds from Grammars

src/main/scala/leon/grammars/DepthBoundedGrammar.scala

src/main/scala/leon/grammars/EmbeddedGrammar.scala

src/main/scala/leon/grammars/Empty.scala

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

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

src/main/scala/leon/grammars/NonTerminal.scala

src/main/scala/leon/grammars/Or.scala

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

src/main/scala/leon/grammars/SizeBoundedGrammar.scala

src/main/scala/leon/solvers/z3/FairZ3Solver.scala

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

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

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

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

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