Add tokio-buf and a BufStream trait (#611)

The `BufStream` trait provides an improved API for working with
asynchronous streams of bytes compared to `Stream<Item = [u8]>`

tokio-buf/src/buf_stream/str.rs

@@ -0,0 +1,39 @@
+use BufStream;
+use buf_stream::errors::internal::Never;
+
+use futures::Poll;
+
+use std::io;
+use std::mem;
+
+impl BufStream for String {
+    type Item = io::Cursor<Vec<u8>>;
+    type Error = Never;
+
+    fn poll_buf(&mut self) -> Poll<Option<Self::Item>, Self::Error> {
+        if self.is_empty() {
+            return Ok(None.into());
+        }
+
+        let bytes = mem::replace(self, Default::default()).into_bytes();
+        let buf = io::Cursor::new(bytes);
+
+        Ok(Some(buf).into())
+    }
+}
+
+impl BufStream for &'static str {
+    type Item = io::Cursor<&'static [u8]>;
+    type Error = Never;
+
+    fn poll_buf(&mut self) -> Poll<Option<Self::Item>, Self::Error> {
+        if self.is_empty() {
+            return Ok(None.into());
+        }
+
+        let bytes = mem::replace(self, Default::default()).as_bytes();
+        let buf = io::Cursor::new(bytes);
+
+        Ok(Some(buf).into())
+    }
+}