buf: Add BufStreamExt trait and add a core feature (#897)

This change adds an extension trait to `BufStream` and puts the core
trait behind a feature flag for optional use.

This mainly adds the additional functions in an extension trait to
allow the user to select if they want just the core trait or the fully
featured version. Now the user can add the core feature to _not_
include the extension trait. By deafult, this feature is disabled.

tokio-buf/src/errors.rs

@@ -0,0 +1,35 @@
+//! Error types
+
+#[cfg(feature = "ext")]
+pub use ext::CollectError;
+#[cfg(feature = "ext")]
+pub use ext::CollectVecError;
+#[cfg(feature = "ext")]
+pub use ext::LimitError;
+
+// Being crate-private, we should be able to swap the type out in a
+// backwards compatible way.
+pub(crate) mod internal {
+    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 {}
+        }
+    }
+}