Generate AstNode accessors via astgen

Introduce the @astgen directives parsed by astgen, currently used for
the generation child node (operand) accessors. Please see the updated
internal documentation for details.

src/V3VariableOrder.cpp

@@ -188,7 +188,7 @@ class VariableOrder final {
                 stmtsp->unlinkFrBackWithNext();
                 AstNode::addNext<AstNode, AstNode>(firstp, stmtsp);
             }
-            modp->addStmtp(firstp);
+            modp->addStmtsp(firstp);
         }
     }