Add a task set

[MikailBag]

Motivation

Closes #3903

Solution

I selected a straightforward approach for the first version. However, try_poll_next_finished implementation should be definitely optimized for a large amounr of tasks, because currently, TaskSet is effectively quadratic.

Also, I wonder if moving spawn method to a clone-able TaskSetSpawner type makes more sense.

[Darksonn]

We are going to want an implementation that is not quadratic. Also, the mutex is unnecessary.

[Darksonn]

And I don't think we want a TaskSetSpawner, but that is a discussion for the issue.

[MikailBag]

We are going to want an implementation that is not quadratic

Ok, I'm going to write one.

I have the following in mind (I think it is similar to FuturesUnordered):

1. Handles are stored in a linked list on the top of Vec, and we separately keep track of holes.
2. Spawning is straightforward.
3. Polling os more complex. For each task we create a custom waker which knows the id of the task (i.e. position in vector). When woken, waker stores this id in a wake queue. Task is also placed into wake queue just after it is spawned

I think this will achieve desired linear run time. Does it sound fine?

[Darksonn]

In general it is dangerous to start working on a PR before we have finished discussing the design. You risk wasting your time on something that doesn't end up being the implementation we end up wanting.

[Darksonn]

Closing since this was implemented in #4335 instead.