Control number of wakes in benchmark streams

[yoshuawuyts]

As mentioned in #80 (comment) our benchmark streams basically just yield once and then stop. Instead we should be able to control how often they yield before halting. In order to do this we'll need to rework our types a bit to support it.

This should make it easier to plot the differences using criterion. Right now our measurements of the difference between select! and stream::merge are fairly static. But if we can control the number of iterations we should get a better picture of what it's like when working with longer-lived streams.