Expose Codec via an unstable flag (#49)

Exposes `Codec` using an unstable flag. This is useful for testing.

tests/support/src/codec.rs

@@ -0,0 +1,64 @@
+#[macro_export]
+macro_rules! assert_closed {
+    ($transport:expr) => {{
+        assert_eq!($transport.poll().unwrap(), None.into());
+    }}
+}
+
+#[macro_export]
+macro_rules! poll_data {
+    ($transport:expr) => {{
+        use h2::frame::Frame;
+        use futures::Async;
+
+        match $transport.poll() {
+            Ok(Async::Ready(Some(Frame::Data(frame)))) => frame,
+            frame => panic!("expected data frame; actual={:?}", frame),
+        }
+    }}
+}
+
+#[macro_export]
+macro_rules! raw_codec {
+    (
+        $(
+            $fn:ident => [$($chunk:expr,)+];
+        )*
+    ) => {{
+        let mut b = $crate::mock_io::Builder::new();
+
+        $({
+            let mut chunk = vec![];
+
+            $(
+                $crate::codec::Chunk::push(&$chunk, &mut chunk);
+            )+
+
+            b.$fn(&chunk[..]);
+        })*
+
+        $crate::Codec::new(b.build())
+    }}
+}
+
+pub trait Chunk {
+    fn push(&self, dst: &mut Vec<u8>);
+}
+
+impl Chunk for u8 {
+    fn push(&self, dst: &mut Vec<u8>) {
+        dst.push(*self);
+    }
+}
+
+impl<'a> Chunk for &'a [u8] {
+    fn push(&self, dst: &mut Vec<u8>) {
+        dst.extend(*self)
+    }
+}
+
+impl<'a> Chunk for &'a str {
+    fn push(&self, dst: &mut Vec<u8>) {
+        dst.extend(self.as_bytes())
+    }
+}