ZipLatest

Combine the elements of 2 streams into a stream of tuples, picking always the latest element of each.

A Zip has a left and a right input port and one out port

'''Emits when''' all of the inputs have at least an element available, and then each time an element becomes available on either of the inputs

'''Backpressures when''' downstream backpressures

'''Completes when''' any upstream completes

'''Cancels when''' downstream cancels