Make DOMString a newtype around String, rather than a typedef.

This should make it somewhat easier to experiment with alternative
representations in the future. To reduce churn, this commit leaves the String
field public, though.

Also, this will allow us to use the default String type to represent the IDL
USVString type, which explicitly forbids unpaired surrogates, ans as such is
a better match to the Rust String type.

components/devtools_traits/lib.rs

@@ -102,22 +102,22 @@ pub struct AttrInfo {
 #[derive(Deserialize, Serialize)]
 pub struct NodeInfo {
     pub uniqueId: String,
-    pub baseURI: String,
+    pub baseURI: DOMString,
     pub parent: String,
     pub nodeType: u16,
-    pub namespaceURI: String,
-    pub nodeName: String,
+    pub namespaceURI: DOMString,
+    pub nodeName: DOMString,
     pub numChildren: usize,
 
-    pub name: String,
-    pub publicId: String,
-    pub systemId: String,
+    pub name: DOMString,
+    pub publicId: DOMString,
+    pub systemId: DOMString,
 
     pub attrs: Vec<AttrInfo>,
 
     pub isDocumentElement: bool,
 
-    pub shortValue: String,
+    pub shortValue: DOMString,
     pub incompleteValue: bool,
 }