Revisit the iterator protocol and add support to yield.

[rainwoodman]

The iterator protocol in Python is the basis for async calls. I'd like to add support to this.

• We need to elevate the integer interface to an object based iterator interface. Probably need to add a new basic type for iterators; then there will be subtypes for list, dict, string, and everything else.

• The cur variable in a dict shall probably be moved out to that iterator.

• To support generator / yield, the grammar needs to be modified. We need a way to mark a function as a generator. Detecting yield may work. Then we need to modify the codegen to create a generator object when the code is executed. Not sure how yet. Get the iterator done first.