Import the util crate as util rather than servo_util.

This used to conflict with the util crate from the standard library, which
has long since been removed.

The import in layout has not been changed because of a conflict with the
util mod there.

components/script/dom/htmltablesectionelement.rs

@@ -14,7 +14,7 @@ use dom::node::{Node, NodeTypeId};
 use dom::virtualmethods::VirtualMethods;
 
 use cssparser::RGBA;
-use servo_util::str::{self, DOMString};
+use util::str::{self, DOMString};
 use std::cell::Cell;
 
 #[dom_struct]