Stop using Object.defineProperty to define exports in lib/types.js.

These properties get copied onto the require("ast-types") exports object
in main.js, anyway, so it's not really important that they be
non-enumerable, non-writable, non-configurable, or whatever.

I'm very proud to say that I did this entire conversion, preserving
comments and all, using a script based on recast.visit:
https://github.com/benjamn/recast/pull/99