folly/futures: replace MoveWrappers with generalised lambda capture

folly/futures: replace MoveWrappers with generalised lambda capture

Summary:Now that folly::Future uses folly::Function, we can use non-copyable
callbacks. That allows us to get rid of folly::MoveWrapper in the
implementaion.

This diff also enforces perfect forwarding in the implementation of
folly::Future, thereby reducing the number of times that a callable
that is passed to Future::then et al. gets passed by value.

Before folly::Function, Future::then(callback) has invoked the move
constructor of the callback type 5 times for small callback objects
(fitting into the in-place storage inside folly::detail::Core) and
6 times for large callback objects. This has been reduced to 5 times
in all cases with the switch to UniqueFunction. This diff reduces it
to 2 times.

Reviewed By: yfeldblum

Differential Revision: D2976647

fb-gh-sync-id: 9da470d7e9130bd7ad8af762fd238ef9a3ac5892
fbshipit-source-id: 9da470d7e9130bd7ad8af762fd238ef9a3ac5892