Make when_all_ready return task<tuple<expected<T>...>> about cppcoro HOT 1 OPEN

I've decided instead to have when_all_ready return an aggregate of when_all_task<T> objects that encapsulates the coroutine that was used to await each of the input awaitables.

See commit c2576ad for the implementation of this (currently on the generic_ops branch).

The benefit of this approach is that we don't need to worry about potential exceptions in copying the results into separate expected<T> object or allocate a separate std::vector<expected<T>> for holding results (which could also potentially fail).

The when_all_task<T>::result() method already lets the caller obtain either a value or rethrow the exception. It can just return a reference to the exact object that was returned from the co_await expression. If the caller then wants to make a copy of the result then they can do so.

Currently, the when_all_task<T> class lives in the cppcoro::detail namespace.
If it's going to be part of the public API for cppcoro we should probably move it out of the detail namespace.

from cppcoro.