Implement nested list comprehensions about mypy

[etrepum]

It seems like the representation here should change to be more similar to how it is in Python's AST

expr = …
         | ListComp(expr elt, comprehension* generators)
         | SetComp(expr elt, comprehension* generators)
         | DictComp(expr key, expr value, comprehension* generators)
         | GeneratorExp(expr elt, comprehension* generators)
         | …

comprehension = (expr target, expr iter, expr* ifs)

[etrepum]

An additional bug here is that only at most one if is allowed

[JukkaL]

See this commit by Eric Price: d3ce840

[etrepum]

Ah, didn't realize there was work happening on a branch. I don't think this fixes #324 but might have a conflict. Is this getting merged any time soon? I can rebase when that happens.

[JukkaL]

I'm probably going to merge the branch later today, assuming I don't find any critical bugs. I'll write a comment when it happens.

[JukkaL]

This was just merged. Closing the issue.