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/str.rs

@@ -1,4 +1,4 @@
-use errors::internal::Never;
+use never::Never;
 use BufStream;
 
 use futures::Poll;