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/transform.js

@@ -262,11 +262,7 @@ TreeTransformer.prototype = new TreeWalker;
     });
 
     _(AST_TemplateString, function(self, tw) {
-        for (var i = 0; i < self.segments.length; i++) {
-            if (!(self.segments[i] instanceof AST_TemplateSegment)) {
-                self.segments[i] = self.segments[i].transform(tw);
-            }
-        }
+        self.segments = do_list(self.segments, tw);
     });
 
     _(AST_PrefixedTemplateString, function(self, tw) {