Add ratio support

[Jeaye Wilkerson]

Clojure supports 3/4 as a ratio. ClojureScript supports this at the reader level, but not at the object level. It will just read it as (/ 3 4), whereas Clojure has a specific ratio object.

For jank, let's follow ClojureScript here until we have a reason to add a ratio object.

Official docs on this: https://clojure.org/reference/data_structures#_ratio