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/lib.rs

@@ -24,14 +24,13 @@ extern crate log;
 
 extern crate string;
 
-pub mod client;
-pub mod error;
+mod error;
+mod codec;
 mod hpack;
 mod proto;
 mod frame;
+
+pub mod client;
 pub mod server;
 
-pub use error::{ConnectionError, Reason};
-
-// TODO: remove if carllerche/http#90 lands
-pub type HeaderMap = http::HeaderMap<http::header::HeaderValue>;
+pub use error::{Error, Reason};