Integrate futures-stable into futures crate.

[withoutboats]

futures-stable provides adapters to work with futures that contain
internal self-references.

futures-stable only works on nightly. A new nightly feature is
added to futures to turn futures-stable on. The futures-stable
items are re-exported from the stable submodule.

Also on nightly only, Future and Stream are implemented for
PinBoxes of futures and streams.

[withoutboats]

cc @cramertj

[cramertj]

This looks great! Thanks! i'll merge once travis is happy.

[withoutboats]

Tests are failing because cargo test --all in the workspace root can't figure out that we want pin-api with default features turned off. Trying to figure out how to make that work.

[withoutboats]

Turned nightly off by default in futures-stable, which should get tests to pass (this is unfortunate, since futures-stable does nothing unless nightly is turned on. But futures-stable is unfortunate in general)

[cramertj]

this is unfortunate, since futures-stable does nothing unless nightly is turned on. But futures-stable is unfortunate in general

SGTM. There's a precedent for this in futures-channel, which is entirely cfg'd out when the std feature is disabled.

[withoutboats]

There's a precedent for this in futures-channel, which is entirely cfg'd out when the std feature is disabled.

Yea thats fine, the unfortunate part is that nightly is not a default feature in futures-stable. Ideally, if you depended directly on futures-stable, by default it would have all the nightly-only items in it.

[cramertj]

Yeah, that is annoying.