Spawn and LocalSpawn implementation

[Nazar Mokrynskyi]

I'd like to see Spawn and LocalSpawn trait implementations from futures (now futures-task actually) to be implemented for executors. That would make it possible to use async-executor with generic code that doesn't necessarily care about specific executor being used (if I understand it correctly).

[Taiki Endo]

The current Spawn and LocalSpawn in futures are quite restrictive as a result of trying to support various use cases. So, in many cases, they are not actually used in the ecosystem. rust-lang/futures-rs#1634

I am planning to improve it in futures 0.4, but it will take quite some time before it is released, so I am going to close this for now.