Add ThreadPool::spawn_ok and spawn_obj_ok

[tomaka]

Considering that spawn and spawn_obj never fail on the ThreadPool, it is desirable to add alternative methods that indeed cannot fail to spawn tasks.

I added this _ok suffix, otherwise that would be a breaking change. People's code would suddenly use the inherent method instead of the trait method, and likely get a compilation error because you can't call unwrap() on ().

Now that SpawnError unfortunately no longer contains the Future that failed to spawn, it is no longer possible to properly handle tasks spawn failures.

[cramertj]

Thanks!