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

src/V3ActiveTop.cpp

@@ -35,7 +35,7 @@
 //######################################################################
 // Active class functions
 
-class ActiveTopVisitor : public AstNVisitor {
+class ActiveTopVisitor final : public AstNVisitor {
 private:
     // NODE STATE
     //  Entire netlist