A first pass at updating Tokio to use `std::future`.
Implementations of `Future` from the futures crate are updated to implement
`Future` from std. Implementations of `Stream` are moved to a feature flag.
This commits disables a number of crates that have not yet been updated.
The futures 0.2 crate is not intended for widespread usage. Also, the
futures team is exploring the compat shim route.
If futures 0.3 support is added to Tokio 0.1, then a different
integration route will be explored, making the current code unhelpful.
Gurwinder Singh