Auto merge of #8477 - asajeffrey:opaque-domstring, r=asajeffrey

Opaque DOMString

This patch makes DOMString an opaque wrapper round String (currently it's a transparent wrapper).

The changes are:

* Replacing DOMString(foo) by DOMString::from(foo).
* Replacing foo.0 by String::from(foo).
* Adding functions clear, push_str and extend for in-place mutation of DOMStrings.
* Replacing DOMString by String in other threads (devtools, storage and filereader).
* Making DOMString implement !Send.
* Removing the pub attribute from the contents of DOMString.

This enables experimenting with other string representations in the DOM.

<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/8477)
<!-- Reviewable:end -->

components/script/dom/dedicatedworkerglobalscope.rs

@@ -243,7 +243,7 @@ impl DedicatedWorkerGlobalScope {
 
             {
                 let _ar = AutoWorkerReset::new(global.r(), worker);
-                scope.execute_script(DOMString(source));
+                scope.execute_script(DOMString::from(source));
             }
 
             let reporter_name = format!("worker-reporter-{}", random::<u64>());