Implement LUB on types during forward type annotation

[sgpthomas]

To make our type system more pythonic, we should compute the LUB on types. For example, if we have the list:

l = [1, "2", 3.0]

We want to find the LUB is Any and assign this list the type List[Any].

This is also useful in computing the return type of a function that returns things of multiple types.

There are probably more places that this is useful. I'll add them to this issue as I remember them.