Add sorted maps and sets

[Jeaye Wilkerson]

Clojure has sorted-map, which builds a PersistentTreeMap, and sorted-set, which builds a PersistentTreeSet. immer doesn't support these, but there's some good info here: arximboldi/immer#105

jank will need its own version, but hopefully we can benefit from the stdlib.

Overall, since we have hash maps, array maps, and hash sets, I think the sorted variants are low priority.