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.