consolidate evaluate & reduce_vars (#1505)

- improve marking efficiency
- apply smarter `const` replacement to `var`

fixes #1501

lib/scope.js

@@ -51,7 +51,6 @@ function SymbolDef(scope, index, orig) {
     this.global = false;
     this.mangled_name = null;
     this.undeclared = false;
-    this.constant = false;
     this.index = index;
     this.id = SymbolDef.next_id++;
 };
@@ -156,7 +155,6 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options){
         else if (node instanceof AST_SymbolVar
                  || node instanceof AST_SymbolConst) {
             var def = defun.def_variable(node);
-            def.constant = node instanceof AST_SymbolConst;
             def.init = tw.parent().value;
         }
         else if (node instanceof AST_SymbolCatch) {