Consistently catch exceptions thrown from set_value() calls

[lewissbaker]

The requirement for set_value() implementations was relaxed to allow them to throw exceptions.
However, this now means we need to audit all call-sites to unifex::set_value() to ensure that we catch exceptions and reflect the error back to unifex::set_error().

Some of this could be helped by a unifex::nothrow_set_value() helper that wraps up this pattern for us conditionally based on whether calling set_value() is noexcept or not.

Places where this needs to be done:

• inline_scheduler
• manual_event_loop::schedule() operation
• ready_done_sender
• stop_immediately concrete_receiver

We also need to update pretty much all of these senders to have error_types that indicates that they might send an exception_ptr since they don't yet know whether they will be connected to a receiver that has a potentially-throwing set_value() method.

[ispeters]

I'm doing this for inline_scheduler in the latest iteration of PR #225.