Verifications for beginning, and completing iteration of a traversable / generator.

[jmalloc]

Possible syntax:

$stub = Phony::stub()->does(function () { yield; });
$stub->never()->started();

[ezzatron]

I don't really like started(), too generic. Let's see if we can find a better name.

[ezzatron]

Also, the flip side of this is probably something like consumed() - verifying that a traversable was completely consumed via iteration.