For the stack safety laws make the number of iterations configurable

[alexandru]

The monix.tail.Iterant data type can implement Async right now, which is pretty cool, however the laws cannot be executed on top of Travis, because those jobs are CPU constrained and the build will eventually time out, due to Iterant being a stream and obviously moving much slower in those tests as Task or IO.

So making that number configurable would be cool.

Also due to playing around I discovered bug #139, so this is useful even in the development of cats-effect.

By making it configurable, we can make the default to be set based on the environment, so we can run it on JavaScript as well, prior to this disabled because JavaScript tests are too slow and can timeout on Travis as well.