Hopac 0.0.0.42

Distribute work more eagerly. Name worker threads. Added monadic composition operator `>=>` for jobs. Updated documentation. Tuning streams. Tweaked Promisesc. Starting to benchmark streams. Avoid an allocation when choosing over lazy promises. Added `Stream.ofObservableOnMain`. Added `Async.getMain`. Added `Stream.ofObservableOn` and `Stream.ofObservable`. Added `IObservable<_>.onceAlt`. Renamed `withNack` as `withNackJob` and `wrapAbort` as `wrapAbortJob`. Added `Alt.wrapAbortFun`. Added `Alt.choosy`, which is an optimized version of `Alt.choose` for arrays. Added `Job.Scheduler.bind` for wrapping external asynchronous events. Added `Alt.withNackFun`. Inlineable `withNack` avoids closure allocation. Added experimental support for running async comptations on the main synchronization context, which must be explicitly configured by application code. Added `IObservable<'x>.onceAltOn` extension method for conveniently interfacing Hopac with suitable observables. Added `TopLevel.startDelay` and `TopLevel.queueDelay` for convenience. Renamed `Builder.Join` to `Builder.Plus` and recognized that `Zero? must also be abstract.