io: remove unsafe pin-projections and remove manual Unpin implementations (#1588)

* Removes most pin-projection related unsafe code.

* Removes manual Unpin implementations.
  As references always implement Unpin, there is no need to implement
  Unpin manually.

* Adds tests to check that Unpin requirement does not change accidentally 
  because changing Unpin requirements will be breaking changes.

tokio-io/src/io/write.rs

@@ -21,9 +21,6 @@ where
     Write { writer, buf }
 }
 
-// forward Unpin
-impl<W: Unpin + ?Sized> Unpin for Write<'_, W> {}
-
 impl<W> Future for Write<'_, W>
 where
     W: AsyncWrite + Unpin + ?Sized,
@@ -35,3 +32,14 @@ where
         Pin::new(&mut *me.writer).poll_write(cx, me.buf)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        use std::marker::PhantomPinned;
+        crate::is_unpin::<Write<'_, PhantomPinned>>();
+    }
+}