Introduce ContextShift for Rerunnable

[felixbr]

This relates to issue #157.

Neither Future nor IO provide a way to get a handle on the underlying thread-pool, so the only way to construct a ContextShift instance is to pass a handle to an ExecutionContext explicitly.

@ben-healthforge mentioned in the issue that this "seems wrong", but I currently don't see a way around it. Feel free to comment if you have any ideas :)

monix.eval.Task is worth mentioning which provides Task.deferAction { implicit scheduler => ... } to get a handle on the underlying Scheduler and I've read this will probably make it into cats-effect 3.0.
For the moment I'd go with this implementation, which is closer to the one for IO.

In the tests I've tried to check correct context-switching for common scenarios like calling into a library like fs2 or http4s from a Finagle/Finatra application (which is based on Future). Hope this makes sense.

[travisbrown]

This looks reasonable to me. Do you think it's ready to merge, @felixbr?

[felixbr]

From my side it's ready. I just waited for feedback from you or @ben-healthforge who initially opened the related issue 🙂