libstd: refactor future, remove get_ref, remove ~ indirection.

[graydon]

Nothing too substantial, just tidying stuff up that erickt found.

r? @erickt or @brson perhaps?

[brson]

Why remove get_ref?

[nikomatsakis]

I would definitely keep get_ref() and lose with(). In general, I think that the with() pattern is better written using lifetimes and immutable pointers. In this case, we have to rely on some unsafe code to get it to type check, but the interface is safe.

[graydon]

The removal of get_ref grew out of trying to make #4020 work, but as I commented in that bug I think the idea of a "resettable future" is itself nonsensical and should be reformed to use pipe::oneshot. I'll update this to leave get_ref in and remove with and, I think, close the other one. Agreed?

[graydon]

landed in a55ea48