Constraint generation can be generalized to remove `Expect::Collection` construct

[JSAbrahams]

Summary of current issue

I think this construct is overly specific and adds unnecessary logic. For a collection, it is relevant that:

• It has an iterator method: __iter__().
• That said iterator method returns an iterator, and we can get the type of the collection then by checking the type of __next__.
• The return type of __next__ then determines the type of the lookup.

For tuples, there is TrueName logic to deal with such constructs. For literal collections (i.e. defined to top level such as def x := {2,3,4}), we can have helper methods which constraint this:

• Iterator method returns a class, and we can pass generics to this class to determine the type of the __next__ method. Then we can proceed as above.
• If we don't know the set type at the generation site, we can probably use a temporary variable.

[JSAbrahams]

Hm seeing as this would greatly help with the dictionary implementation in #438 , I'm moving this to the 0.3.6 milestone.