a00e98bad9
* It enables subtyping relationship for keys and elements, allowing type-safe and concise manipulations on the context when complex hierarchies of elements (e.g. implementors of ContinuationInterceptor) are present
* It unblocks possibility to implement suspendCancellableCoroutine in stdlib
* Implementing proper polymorphic get and minusKey in AbstractCoroutineContextElement is impossible because it will introduce source-incompatible change
* ContinuationInterceptor get and minusKey are adjusted in a completely backwards-compatible way