Refactor errors (#46)

This patch does a bunch of refactoring, mostly around error types, but it also
paves the way to allow `Codec` to be used standalone.

* `Codec` (and `FramedRead` / `FramedWrite`) is broken out into a codec module.
* An h2-codec crate is created that re-exports the frame and codec modules.
* New error types are introduced in the internals:
  * `RecvError` represents errors caused by trying to receive a frame.
  * `SendError` represents errors caused by trying to send a frame.
  * `UserError` is an enum of potential errors caused by invalid usage
    by the user of the lib.
  * `ProtoError` is either a `Reason` or an `io::Error`. However it doesn't
    specify connection or stream level.
  * `h2::Error` is an opaque error type and is the only error type exposed
    by the public API (used to be `ConnectionError`).

There are misc code changes to enable this as well. The biggest is a new "sink"
API for `Codec`. It provides buffer which queues up a frame followed by flush
which writes everything that is queued. This departs from the `Sink` trait in
order to provide more accurate error values. For example, buffer can never fail
(but it will panic if `poll_ready` is not called first).

src/proto/settings.rs

@@ -1,8 +1,7 @@
 use frame;
+use codec::RecvError;
 use proto::*;
 
-use futures::Sink;
-
 #[derive(Debug)]
 pub(crate) struct Settings {
     /// Received SETTINGS frame pending processing. The ACK must be written to
@@ -31,7 +30,7 @@ impl Settings {
     pub fn send_pending_ack<T, B, C, P>(&mut self,
                                         dst: &mut Codec<T, B>,
                                         streams: &mut Streams<C, P>)
-        -> Poll<(), ConnectionError>
+        -> Poll<(), RecvError>
         where T: AsyncWrite,
               B: Buf,
               C: Buf,
@@ -40,13 +39,17 @@ impl Settings {
         trace!("send_pending_ack; pending={:?}", self.pending);
 
         if let Some(ref settings) = self.pending {
-            let frame = frame::Settings::ack();
-
-            if let AsyncSink::NotReady(_) = dst.start_send(frame.into())? {
+            if !dst.poll_ready()?.is_ready() {
                 trace!("failed to send ACK");
                 return Ok(Async::NotReady);
             }
 
+            // Create an ACK settings frame
+            let frame = frame::Settings::ack();
+
+            // Buffer the settings frame
+            dst.buffer(frame.into()).ok().expect("invalid settings frame");
+
             trace!("ACK sent; applying settings");
 
             dst.apply_remote_settings(settings);