Add rule to prefer weak captures over unowned captures

[calda]

Summary

This PR proposes a new rule to prefer using weak captures over unowned captures.

Reasoning

unowned captures are unsafe because they will cause the application to crash if the referenced object has been deallocated. weak captures are safer because they require the author to explicitly consider and handle the case where the referenced object no longer exists.

// WRONG: Crashes if `planet` has been deallocated when the closure is called.

spaceship.travel(to: planet, onArrival: { [unowned planet] in
  planet.colonize()
})

spaceship.travel(to: planet, nextDestination: { [unowned planet] in
  planet.moons.first ?? planet.sun
})

// RIGHT: Explicitly handles case where `planet` has been deallocated.

spaceship.travel(to: planet, onArrival: { [weak planet] in
  guard let planet else { return }
  planet.colonize()
})

// For closures that return a non-optional value, you could either 
// use `fatalError` to avoid having to return anything:
spaceship.travel(to: planet, nextDestination: { [weak planet] in
  guard let planet else {
    fatalError("Planet was unexpectedly deallocated before reached by spaceship")
  }
  
  return planet.moons.first ?? planet.sun
})

// Or you could return a placeholder value with an optional `assertionFailure`:
spaceship.travel(to: planet, nextDestination: { [weak planet] in     
  guard let planet else {
    assertionFailure("Planet was unexpectedly deallocated before reached by spaceship")
    return Planet()
  }
  
  return planet.moons.first ?? planet.sun
})

Please react with 👍/👎 if you agree or disagree with this proposal.

[jqsilver]

Requesting changes just to get this back into your queue.