Internals: Use and enforce class final for ~5% performance boost.

src/V3Cast.cpp

@@ -49,7 +49,7 @@
 //######################################################################
 // Cast state, as a visitor of each AstNode
 
-class CastVisitor : public AstNVisitor {
+class CastVisitor final : public AstNVisitor {
 private:
     // NODE STATE
     // Entire netlist: