collections: Implement Eq for DList, RingBuf, TreeMap, TreeSet
collections: Implement Ord for DList, RingBuf, TreeMap, TreeSet
collections: Implement FromIterator/Extendable for BitvSet
auto merge of #16177 : nham/rust/collections_15294_eq_ord, r=alexcrichton

This implements:

 - Eq and Ord for DList, RingBuf, TreeMap and TreeSet
 - FromIterator and Extendable for BitvSet

cc #15294