Bug 798624 - Specialize low-level character access to JSStableString; r=luke, rs=Waldo

Implements JSStableString::chars() and pushes StableCharPtr into the interface
at several key choke-points. This will solidly enforce the requirement that we
have uninlined jschar*s in places that we may GC with a jschar* on the stack.

js/src/jsatom.cpp

@@ -312,7 +312,7 @@ js::AtomizeString(JSContext *cx, JSString *str, InternBehavior ib)
     if (!stable)
         return NULL;
 
-    const jschar *chars = stable->chars();
+    const jschar *chars = stable->chars().get();
     size_t length = stable->length();
     JS_ASSERT(length <= JSString::MAX_LENGTH);
     return AtomizeInline(cx, &chars, length, ib);