Split ConnectionUpgrade. (#642)

Introduce `InboundUpgrade` and `OutboundUpgrade`.

core/src/either.rs

@@ -18,11 +18,42 @@
 // FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+use crate::{muxing::{Shutdown, StreamMuxer}, Multiaddr};
 use futures::prelude::*;
-use muxing::{Shutdown, StreamMuxer};
-use std::io::{Error as IoError, Read, Write};
+use std::{fmt, io::{Error as IoError, Read, Write}};
 use tokio_io::{AsyncRead, AsyncWrite};
-use Multiaddr;
+
+#[derive(Debug, Copy, Clone)]
+pub enum EitherError<A, B> {
+    A(A),
+    B(B)
+}
+
+impl<A, B> fmt::Display for EitherError<A, B>
+where
+    A: fmt::Display,
+    B: fmt::Display
+{
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            EitherError::A(a) => a.fmt(f),
+            EitherError::B(b) => b.fmt(f)
+        }
+    }
+}
+
+impl<A, B> std::error::Error for EitherError<A, B>
+where
+    A: fmt::Debug + std::error::Error,
+    B: fmt::Debug + std::error::Error
+{
+    fn cause(&self) -> Option<&dyn std::error::Error> {
+        match self {
+            EitherError::A(a) => a.cause(),
+            EitherError::B(b) => b.cause()
+        }
+    }
+}
 
 /// Implements `AsyncRead` and `AsyncWrite` and dispatches all method calls to
 /// either `First` or `Second`.