OR-5284 Schedulers

[crazymanish]

Context

• Close ticket: [w32(6august-12august)] Schedulers: Practices/exercises #47

In this PR

• Base branch of below PRs:
• OR-5283 Schedulers :: Operators for scheduling #137
• OR-5283 Schedulers:: Scheduler implementations:: ImmediateScheduler #138
• OR-5284 Schedulers:: Scheduler implementations:: RunLoop DispatchQueue OperationQueue #139

Highlights

• A Scheduler defines the execution context for a piece of work.
• Apple‘s operating systems offer a rich variety of tools to help you schedule code execution.
• Combine tops these schedulers with the Scheduler protocol to help you pick the best one for the job in any given situation.
• Every time you use receive(on:), further operators in your publisher execute on the specified scheduler. That is, unless they themselves take a Scheduler parameter!