futures::chain

futures::chain

Summary:
`futures::chain<A,Z>(a, b, c, d, e, f, ..., z)` where `a` is a callback suitable for `Future<A>::then` and `z` is a callback suitable for `Future<Y>::then<Z>`.

This will be important, at least in the background, for the new `via`. It will probably also be useful to some user sometime.

I imagine this will be tweaked over time if people find magic ways to get it to deduce the types better. But this works and it's not *too* much trouble to specify A and Z.

Test Plan: unit

Reviewed By: davejwatson@fb.com

Subscribers: trunkagent, exa, folly-diffs@, yfeldblum, jsedgwick

FB internal diff: D1831073

Tasks: 6048744

Signature: t1:1831073:1423259292:711be0e047a2acb706fd74e529d5d5fb6abda566