Update Tokio to use std::future. (#1120)

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.

tokio-executor/src/enter.rs

@@ -1,9 +1,8 @@
-use futures::{self, Future};
 use std::cell::{Cell, RefCell};
 use std::error::Error;
 use std::fmt;
+use std::future::Future;
 use std::marker::PhantomData;
-use std::prelude::v1::*;
 
 thread_local!(static ENTERED: Cell<bool> = Cell::new(false));
 
@@ -65,8 +64,25 @@ pub fn enter() -> Result<Enter, EnterError> {
 impl Enter {
     /// Blocks the thread on the specified future, returning the value with
     /// which that future completes.
-    pub fn block_on<F: Future>(&mut self, f: F) -> Result<F::Item, F::Error> {
-        futures::executor::spawn(f).wait_future()
+    pub fn block_on<F: Future>(&mut self, mut f: F) -> F::Output {
+        use crate::park::{Park, ParkThread};
+        use std::pin::Pin;
+        use std::task::Context;
+        use std::task::Poll::Ready;
+
+        let park = ParkThread::new();
+        let waker = park.unpark().into_waker();
+        let mut cx = Context::from_waker(&waker);
+
+        // `block_on` takes ownership of `f`. Once it is pinned here, the original `f` binding can
+        // no longer be accessed, making the pinning safe.
+        let mut f = unsafe { Pin::new_unchecked(&mut f) };
+
+        loop {
+            if let Ready(v) = f.as_mut().poll(&mut cx) {
+                return v;
+            }
+        }
     }
 }