Add immer-based hash map object

[Jeaye Wilkerson]

Now that #27 is done, we can add a hash map with transients into jank.

• Add new hash map object
• Add promotion from array map to hash map, based on size
• Codegen hash maps by default, based on size of map (16 or more in Clojure)
• Add hash-map function to clojure.core
• Ensure both map types are map?

This is blocked on #37, which is changing the whole object model. Makes sense to only do this afterward.