feat(lib): update to std::future::Future

BREAKING CHANGE: All usage of async traits (`Future`, `Stream`,
`AsyncRead`, `AsyncWrite`, etc) are updated to newer versions.

src/common/mod.rs

@@ -1,3 +1,12 @@
+macro_rules! ready {
+    ($e:expr) => (
+        match $e {
+            ::std::task::Poll::Ready(v) => v,
+            ::std::task::Poll::Pending => return ::std::task::Poll::Pending,
+        }
+    )
+}
+
 mod buf;
 pub(crate) mod drain;
 pub(crate) mod exec;
@@ -10,4 +19,12 @@ pub(crate) use self::buf::StaticBuf;
 pub(crate) use self::exec::Exec;
 pub(crate) use self::lazy::{lazy, Started as Lazy};
 pub use self::never::Never;
-pub(crate) use self::task::YieldNow;
+pub(crate) use self::task::Poll;
+
+// group up types normally needed for `Future`
+pub(crate) use std::{
+    future::Future,
+    marker::Unpin,
+    pin::Pin,
+};
+