io: move io helpers back into tokio-io (#1377)

Utilities are made optional with a feature flag.

tokio-io/src/io/write.rs

@@ -0,0 +1,37 @@
+use crate::AsyncWrite;
+use std::future::Future;
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// A future to write some of the buffer to an `AsyncWrite`.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct Write<'a, W: ?Sized> {
+    writer: &'a mut W,
+    buf: &'a [u8],
+}
+
+/// Tries to write some bytes from the given `buf` to the writer in an
+/// asynchronous manner, returning a future.
+pub(crate) fn write<'a, W>(writer: &'a mut W, buf: &'a [u8]) -> Write<'a, W>
+where
+    W: AsyncWrite + Unpin + ?Sized,
+{
+    Write { writer, buf }
+}
+
+// forward Unpin
+impl<'a, W: Unpin + ?Sized> Unpin for Write<'a, W> {}
+
+impl<W> Future for Write<'_, W>
+where
+    W: AsyncWrite + Unpin + ?Sized,
+{
+    type Output = io::Result<usize>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<usize>> {
+        let me = &mut *self;
+        Pin::new(&mut *me.writer).poll_write(cx, me.buf)
+    }
+}