- Change fmap, schedule_on, resume_on, make_shared_task work with
arbitrary awaitables.
- Add make_task to wrap an arbitrary awaitable in a task<T>.
- Remove specialisation of fmap() for task<T>, shared_task<T> as
the generalised version obsoletes it.
The await_suspend() method of operator co_await() now returns bool
instead of void, to allow it to resume execution without suspending
if the task completes synchronously. However, this unfortunately
requires use of std::atomic operations to decide a potential race
between the awaiter suspending and the coroutine running to
completion.
This avoids a stack-overflow error when a coroutine awaits a
long sequence of task<T> values that complete synchronously.
eg. in a loop.
