Add support for MyPy types in data blocks

[jonathanplatzer]

It would be great if MyPy types would be allowed in data blocks.

If I understand the way that the immutability of the data block is achieved, by using a _coconut.collections.namedtuple, this could be easily implemented by using MyPys own NamedTuple.

So this

data User(uid: int, username: str, friends: List[User])
   ...

could be compiled to this:

from typing import NamedTuple

class User(NamedTuple("User", [('uid', int), 
                               ('username', str), 
                               ('friends', List[User])])):
   ...

[evhub]

@jonathanplatzer That's a great idea! There is the issue that typing doesn't exist on old Python versions, but I think I can get around that via stub file magic (real _coconut.NamedTuple uses collections.namedtuple, but stub file _coconut.NamedTuple uses typing.NamedTuple).

[evhub]

@jonathanplatzer This is now live in coconut-develop! Just pip install coconut-develop to test it out.