Test for Enumerator as external iterator, and explicit Fiber communication

[funny-falcon]

While external iteration is not very useful, it could unintentionally appear with zip method.
Also, no one prevents from using Fiber.yield+Fiber.resume directly. It is sometimes really useful.

Test for #23

[coveralls]

Coverage increased (+0.5%) to 97.297% when pulling 966415c on funny-falcon:fiber_test into 68db286 on socketry:master.

[coveralls]

Coverage increased (+0.5%) to 97.297% when pulling 966415c on funny-falcon:fiber_test into 68db286 on socketry:master.

[coveralls]

Coverage increased (+0.5%) to 97.297% when pulling 966415c on funny-falcon:fiber_test into 68db286 on socketry:master.

[coveralls]

Coverage increased (+0.5%) to 97.297% when pulling 966415c on funny-falcon:fiber_test into 68db286 on socketry:master.

[ioquatix]

I am experimenting with this and the thread-selector PR.

[ioquatix]

I will merge this and experiment.