Map types

[dahlia]

Although the runtime library already had had serializer and deserializer for maps, they weren't correctly implemented according to the spec. Map should be encoded as an array of objects e.g. [{"key": ..., "value": ...}] since keys can be more than primitive types, but it had encoded map as an object e.g. {...: ...}.

Also, as Python doesn't offset immutable dict, I implemented a simple immutable dict. Though its name is Map to follow Nirum's own terms.

[kanghyojun]

이거 머지하면 0.3-maintanence 마스터에 머지하고 닫아야겠네요