buf: misc polish (#924)

- Rename feature flag `util`.
- Rename module `util`
- Move `error` module into `util`.
- Move `BufStream` impls into dedicated file.

tokio-buf/src/u8.rs

@@ -0,0 +1,66 @@
+use BufStream;
+use never::Never;
+use bytes::{Bytes, BytesMut};
+use futures::Poll;
+use std::io;
+
+impl BufStream for Vec<u8> {
+    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());
+        }
+
+        poll_bytes(self)
+    }
+}
+
+impl BufStream for &'static [u8] {
+    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());
+        }
+
+        poll_bytes(self)
+    }
+}
+
+impl BufStream for Bytes {
+    type Item = io::Cursor<Bytes>;
+    type Error = Never;
+
+    fn poll_buf(&mut self) -> Poll<Option<Self::Item>, Self::Error> {
+        if self.is_empty() {
+            return Ok(None.into());
+        }
+
+        poll_bytes(self)
+    }
+}
+
+impl BufStream for BytesMut {
+    type Item = io::Cursor<BytesMut>;
+    type Error = Never;
+
+    fn poll_buf(&mut self) -> Poll<Option<Self::Item>, Self::Error> {
+        if self.is_empty() {
+            return Ok(None.into());
+        }
+
+        poll_bytes(self)
+    }
+}
+
+fn poll_bytes<T: Default>(buf: &mut T) -> Poll<Option<io::Cursor<T>>, Never> {
+    use std::mem;
+
+    let bytes = mem::replace(buf, Default::default());
+    let buf = io::Cursor::new(bytes);
+
+    Ok(Some(buf).into())
+}