Reimplement scheduler affinity (#527)

This diff changes how `unifex::task<>` implements *Scheduler* affinity to work more like `folly::coro::Task<>`.  What Folly calls `co_viaIfAsync`, this diff calls `with_scheduler_affinity`, which is a new CPO.

`with_scheduler_affinity` is a *Sender* algorithm that maps the given *Sender* to another *Sender* that must meet new postconditions if the *Receiver* that it's connected to meets certain preconditions.  The new preconditions are:
 - the eventual *Receiver* must provide a "current *Scheduler*";
 - the result of `with_scheduler_affinity` must be started on the *Receiver's* current *Scheduler*; and
 - stop requests delivered to the new *Sender* must be delivered on the *Receiver's* current *Scheduler*.

The *Sender* returned from `with_scheduler_affinity` must complete on its *Receiver's* current *Scheduler* (i.e. must complete where it was started) so long as all the above preconditions are met.

`with_scheduler_affinity` has two default implementations for a _Sender_ type `S`:
 - if `sender_traits<S>::is_always_scheduler_affine` is `true`, the default is the identity;
 - if `sender_traits<S>::is_always_scheduler_affine` is `false`, the default is, essentially, a `typed_via` back to the correct scheduler.
 
Any _Sender_ can customize `with_scheduler_affinity`, regardless of its `is_always_scheduler_affine` property.

include/unifex/at_coroutine_exit.hpp

@@ -318,6 +318,8 @@ namespace _at_coroutine_exit {
   } at_coroutine_exit{};
 } // namespace _at_coroutine_exit
 
+// TODO: verify that `at_coroutine_exit()` can't be used to break scheduler
+//       affinity by running an async task that reschedules
 using _at_coroutine_exit::at_coroutine_exit;
 
 } // namespace unifex