deserialize_meta won't union properly

[kanghyojun]

https://gitter.im/spoqa/nirum?at=57dd736133c63ba01a0e09e2

[dahlia]

@admire93 Although PR #35 fixed it to work, we need to refactor the generated code to be conciser and more efficient than now.

• [x] The compiler should generate __nirum_type__ = 'union' # or 'record'/'tag'/'enum'/'boxed' so that the runtime can determine what Nirum type the given Python class is generated from.
• [x] Also, the compiler should generate __nirum_tag_classes__ = {Tag.tag_name: TagName, ...} so that the runtime can lookup the corresponding Python class by the given tag name in O(1). The current implementation loops over cls.__subclasses__() which is O(n).