Implement Index for RingBuf

This also deprecates RingBuf::get. Use indexing instead.
Implement Index for HashMap

This also deprecates HashMap::get. Use indexing instead.
Implement Index for TreeMap
Implement Index for SmallIntMap

This also deprecates SmallIntMap::get. Use indexing instead.
Implement Index for TrieMap
auto merge of #16195 : P1start/rust/more-index, r=alexcrichton

Implement `Index` for `RingBuf`, `HashMap`, `TreeMap`, `SmallIntMap`, and `TrieMap`.

If there’s anything that I missed or should be removed, let me know.