collections: Add missing Default impls

Add Default impls for TreeMap, TreeSet, SmallIntMap, BitvSet, DList,
PriorityQueue, RingBuf, TrieMap, and TrieSet.
auto merge of #14769 : tomjakubowski/rust/default-collections, r=huonw

Add `Default` impls for `TreeMap`, `TreeSet`, `SmallIntMap`, `BitvSet`, `DList`,
`PriorityQueue`, `RingBuf`, `TrieMap`, and `TrieSet`.