Add an unsafe method to rayon-core to enable `rayon-futures`

[alexcrichton]

It sounded like rayon, which ideally hits 1.0 soon-ish, will reach 1.0 before the futures crate but the rayon 1.0 release shouldn't publicly depend on unstable crates. In that sense it sounds like a separate crate, rayon-futures, is the way to go for now, but needs support in rayon-core

[cuviper]

#436 made rayon-futures a separate crate, but we still need to decide if we like that core interface.
(still under --cfg rayon_unstable for now)

[cuviper]

That code was removed in #716.