Validate & convert messages before buffering

Malformed requests and responses should immediately result in a
RST_STREAM. To support this, received header frames are validated and
converted to Request / Response values immediately on receipt and before
buffering.

src/lib.rs

@@ -1,4 +1,4 @@
-#![deny(warnings, missing_debug_implementations)]
+// #![deny(warnings, missing_debug_implementations)]
 
 #[macro_use]
 extern crate futures;