feat(lib): switch to non-blocking (asynchronous) IO

BREAKING CHANGE: This breaks a lot of the Client and Server APIs.
  Check the documentation for how Handlers can be used for asynchronous
  events.

src/header/parsing.rs

@@ -137,6 +137,12 @@ define_encode_set! {
     }
 }
 
+impl fmt::Debug for HTTP_VALUE {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("HTTP_VALUE")
+    }
+}
+
 impl Display for ExtendedValue {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         let encoded_value =