Added cancelable

[djspiewak]

The unit test does a pretty good job of demonstrating the use case. Reproducing here in more condensed form:

IO.deferred[Unit] flatMap { latch =>
  val badness = latch.get.uncancelable
  badness.cancelable(latch.complete(()).void
}

Under normal circumstances, the above would be a fancy version of never. The use of cancelable converts this into something which has the natural semantics: canceling the effect completes the Deferred and unlocks the fiber. More usefully, this can be applied to effects constructed with blocking or delay to make them safely cancelable in the case where some state can be mutated to prematurely terminate the effect.

Generalizes #3459, as well as #3374

[armanbilge]

Thanks, this is so much nicer than what I had 😁 🤦

[durban]

Related discussion: #2671.

[SystemFw]

This makes perfect sense to me. In fact, I have a vague memory of double checking this was encodable with uncancelable : the shape of cancelable being added here was the main combinator for cancelation in early versions of cats-effect

[johnynek]

nit: seems you usually use IO.blocking no?