Add unordered_set/unordered_map containers

Add `unordered_set` and `unordered_map` associative containers based on [`std::unordered_set`](http://en.cppreference.com/w/cpp/container/unordered_set) and [`std::unordered_map`](http://en.cppreference.com/w/cpp/container/unordered_map).
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