- Don't use metadata to store ast, env, params data.
- In Clojure, store metadata on the :meta key of the real metadata.
This also allows using any datatype as metadata.
- types: low-level mapping to the implementation language.
- core: functions on types that are exposed directly to mal.
- printer: implementation called by pr-str, str, prn, println.
- env: the environment implementation
- Also, unindent all TCO while loops so that the diff of step4 and
step5 are minimized.
