An API for not populating the exception stacktraces for coroutines that are likely to get cancelled

[dkhalanskyjb]

Some coroutines are very likely to get cancelled a short time after they are launched, making the CancellationException stacktrace calculation into a large portion of the runtime.

Examples:

• This was shared with us: https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:compose/foundation/foundation/src/commonMain/kotlin/androidx/compose/foundation/gestures/MouseWheelScrollable.kt;l=266;drc=640651fd5298977dc63c0d11f646fb4b219a5683
• Launching several coroutines to race with one another (Provide an API to race coroutines ("losers" are cancelled) #2867) if one operation (for example, accessing a cache) is very short-lived.

This calls for an API that allows not populating the stacktraces but instead using a CancellationException object created in advance.

Options:

1. Cancel-site: cancel already can accept custom CancellationException variants, and withTimeout* could also be parameterized to allow this.
2. Throw-site: suspendCancellableContinuation could react to a special CoroutineContextElement specifying that the cancellation exception of the suspended coroutine itself does not need to be instantiated.