Allow lambda with Void return

[etrepum]

Fixes #108

Not entirely sure this is the best way to go, but it passes the tests. Also, I don't understand the type of these visit_* methods. How do they return Type without any return statements?

[JukkaL]

The implementation is okay, though the lambda context check would be better in a utility method.

Currently the Python implicit None return is always accepted (i.e., a return statement is always optional). This probably a bad idea, and we should enforce an explicit return (potentially with a None return value). I added the issue #320 for this.

The visit_* methods are sloppy and should return an explicit None, though even this is somewhat sloppy... expressions and statements could plausibly have separate class hierarchies and visitors, but this would complicate the implementation of visitors that need to traverse arbitrary nodes.