Add stream_id accessors to public API types (#292)

Problem:

Applications may want to access the underlying h2 stream ID for
diagnostics, etc. Stream IDs were not previously exposed in public APIs.

Solution:

Added a new public `share::StreamId` type, which has a more restricted 
API than the internal `frame::StreamId` type. The public API types 
`SendStream`, `RecvStream`, `ReleaseCapacity`, 
`client::ResponseFuture`, and `server::SendResponse` now all have 
`stream_id` methods which return the stream ID of the corresponding 
stream.

Closes #289.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

src/lib.rs

@@ -128,7 +128,7 @@ pub mod server;
 mod share;
 
 pub use error::{Error, Reason};
-pub use share::{SendStream, RecvStream, ReleaseCapacity};
+pub use share::{SendStream, StreamId, RecvStream, ReleaseCapacity};
 
 #[cfg(feature = "unstable")]
 pub use codec::{Codec, RecvError, SendError, UserError};