Add when_all() function for waiting for multiple tasks to complete

[lewissbaker]

No description provided.

[lewissbaker]

Commit 67fea7f adds initial support for a when_all_ready() function that takes a number of tasks and concurrently waits for them all to complete, returning those tasks back in the result without rethrowing any exceptions in the task results.

It also adds support for when_all() which waits until all tasks are ready and then unpacks the results into a value (either tuple or vector depending on the overload). This variation will throw an exception if any of the tasks throws an exception.

Added variadic overloads of these two functions that return std::tuple results.
Also added overloads that take std::vector of homogeneous tasks that return std::vector of results.

Documentation of these functions is still pending.

[lewissbaker]

It would be really handy to add support for when_all() of void-returning task types.
Event better if I can combine void-returning task types with non-void-returning task types.

Perhaps when_all() could translate void-returning tasks into some empty struct (like cppcoro::unit or cppcoro::void_value) so that it can be stored in the std::tuple<> returned by when_all()?

[lewissbaker]

Documentation added in e6355df