Legacy callback based api to AsyncSequence

[jmjauer (Johannes Auer)]

It seems that @Tony_Parker ist already working on a solution:

[Concurrency] YieldingContinuation

Hi everyone, We'd like to start discussion on an addition to the Concurrency library. This new type will be used for adapting existing multiple-callback functions (for example, closures or delegates) into an AsyncSequence-conforming type. We are building upon the Continuation API in SE-300. The existing UnsafeContinuation type is designed to be resumed exactly once. For a sequence, we really want a type which can be sent values multiple times and be queried for a value multiple times. This ty…