Add support for a coroutine task type

[lewissbaker]

To improve the user-experience of sender/receiver we should also provide a coroutine task type that allows users to implement and consume senders by writing a coroutine.

Some basic requirements:

• ability to customize the allocation
• ability to customize the set of receiver queries to be forwarded through to children of the coroutine
• ability to specify a scheduler and ensure that the coroutine always resumes on that scheduler
• ability to automatically propagate stop-requests through the coroutine to awaited child operations
• ability for a task coroutine to complete with a 'stopped' result

Other things to consider:

• ability to schedule async cleanup operations to run upon coroutine exit
• ability to yield an error result without throwing an exception
• issue of stack-overflow when awaiting senders in a loop (see also recursion, inline completion, and stack usage #135 and Propose symmetric transfer for senders (tail senders?) #38)

[lewissbaker]

See also #36