Basic skeleton for DOMException

Spec:
http://dom.spec.whatwg.org/#domexception

This is a subtask for #1542.
auto merge of #1567 : pcwalton/servo/quicksort-selector-matching, r=larsbergstrom

17% improvement in selector matching on the rainbow page.

r? @larsbergstrom
auto merge of #1580 : brunoabinader/servo/dom-exception, r=Ms2ger

Spec:
http://dom.spec.whatwg.org/#domexception

This is a subtask for #1542.
util: Implement a Doug Lea-style concurrent hash table.

This will be used for leaf sets and interned strings.