Add names to tasks

[notgull]

In order to make it easier to debug tasks, it should be possible to give tasks names.

I was thinking of an API like this:

Task::builder().name("my-task").spawn(async { println!("Hello world!"); });

Where, if the name() was occluded, it would be the same as Task::spawn.

[taiki-e]

Sounds good to me.