Add Yokeable::upgrade_output function

[sffc]

I would ideally like to avoid having any unsafety here.

I think all we need here is fn upgrade_output<Y: Yokeable<'static>>(output: Y::Output) -> Y, which internally calls Yokeable::make(). Does that make sense? Yokeable::make() is only scarily unsafe when the lifetimes involved aren't 'static.

Originally posted by @Manishearth in #3467 (comment)

[Manishearth]

This can't live on Yokeable but can be a free function in the crate somewhere.

(I guess it can if you give it a where bound but really I don't think it should anyway)