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/frame/mod.rs

@@ -1,5 +1,4 @@
 use hpack;
-use error::{ConnectionError, Reason};
 
 use bytes::Bytes;
 
@@ -33,6 +32,7 @@ mod head;
 mod headers;
 mod ping;
 mod priority;
+mod reason;
 mod reset;
 mod settings;
 mod stream_id;
@@ -45,6 +45,7 @@ pub use self::head::{Head, Kind};
 pub use self::headers::{Headers, PushPromise, Continuation, Pseudo};
 pub use self::ping::Ping;
 pub use self::priority::{Priority, StreamDependency};
+pub use self::reason::Reason;
 pub use self::reset::Reset;
 pub use self::settings::Settings;
 pub use self::stream_id::StreamId;
@@ -113,15 +114,6 @@ impl<T> fmt::Debug for Frame<T> {
 /// Errors that can occur during parsing an HTTP/2 frame.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum Error {
-    /// A full frame header was not passed.
-    Short,
-
-    /// An unsupported value was set for the flag value.
-    BadFlag,
-
-    /// An unsupported value was set for the frame kind.
-    BadKind,
-
     /// A length value other than 8 was set on a PING message.
     BadFrameSize,
 
@@ -129,19 +121,6 @@ pub enum Error {
     /// length of the payload.
     TooMuchPadding,
 
-    /// The payload length specified by the frame header was shorter than
-    /// necessary for the parser settings specified and the frame type.
-    ///
-    /// This happens if, for instance, the priority flag is set and the
-    /// header length is shorter than a stream dependency.
-    ///
-    /// `PayloadLengthTooShort` should be treated as a protocol error.
-    PayloadLengthTooShort,
-
-    /// The payload length specified by the frame header of a settings frame
-    /// was not a round multiple of the size of a single setting.
-    PartialSettingLength,
-
     /// An invalid setting value was provided
     InvalidSettingValue,
 
@@ -173,14 +152,3 @@ pub enum Error {
     /// Failed to perform HPACK decoding
     Hpack(hpack::DecoderError),
 }
-
-// ===== impl Error =====
-
-impl From<Error> for ConnectionError {
-    fn from(src: Error) -> ConnectionError {
-        match src {
-            // TODO: implement
-            _ => ConnectionError::Proto(Reason::ProtocolError),
-        }
-    }
-}