fix & improve AST_TemplateString (#2410)

- resolve `semicolons:false` ambiguity with tagged literals
- allow `side_effects` to work on template literals
- traverse `AST_TemplateString` properly

fixes #2401

lib/ast.js

@@ -448,10 +448,8 @@ var AST_TemplateString = DEFNODE("TemplateString", "segments", {
     },
     _walk: function(visitor) {
         return visitor._visit(this, function(){
-            this.segments.forEach(function(seg, i){
-                if (i % 2 !== 0) {
-                    seg._walk(visitor);
-                }
+            this.segments.forEach(function(seg){
+                seg._walk(visitor);
             });
         });
     }