POC: plugging the `cancelable` leak

[armanbilge]

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Implements a proof-of-concept for my proposal in https://github.com/typelevel/cats-effect/issues/3474#issuecomment-1459309054.

To re-iterate:

1. IO.Cancelable[+A](ioa: IO[A], cancel: IO[Unit]) is now a primitive operation
2. When you enter a Cancelable, it exposes its cancel operation as part of the fiber.cancel.
3. By leaning into uncancelable semantics, this guarantees that the final result of ioa (be it success, error, or cancellation) will always be propagated.

As such, it plugs the leak.