Clarify contract for dealing with synchronous pipelines

[alexandru]

The contract will change such that:

• only direct references to Continue and Cancel are considered to be synchronous
• in operators we'll deal with optimizations on a case by case basis

Move Ack in monix.execution and introduce these macro-enabled extensions for Future[Ack]:

• def syncOnContinue(callback: => Unit)(implicit s: Scheduler): Self (macro)
• def syncOnCancelOrFailure(callback: => Unit)(implicit s: Scheduler): Self (macro)
• def syncMap(f: Ack => Ack)(implicit s: Scheduler): Future[Ack] (macro)
• def syncFlatMap(f: Ack => Future[Ack])(implicit s: Scheduler): Future[Ack] (macro)
• def syncTryFlatten(implicit r: UncaughtExceptionReporter): Future[Ack] (macro)

The purpose of these is dealing with synchronous execution. These will replace our internal utilities defined in monix.internal.

[alexandru]

Done.