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/never.rs

@@ -0,0 +1,22 @@
+use std::{error, fmt};
+
+/// An error that can never occur
+pub enum Never {}
+
+impl fmt::Debug for Never {
+    fn fmt(&self, _f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {}
+    }
+}
+
+impl fmt::Display for Never {
+    fn fmt(&self, _f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {}
+    }
+}
+
+impl error::Error for Never {
+    fn description(&self) -> &str {
+        match *self {}
+    }
+}