Issue 128: Convert frame::Reason to struct (#142)

Alter frame::Reason to a struct with a single u32 member.
Introduce Constants to the impl for existing Reasons. Change all usage
in the library and its tests to adopt this change,
using the new constants.

src/proto/streams/mod.rs

@@ -23,7 +23,6 @@ use self::state::State;
 use self::store::{Entry, Store};
 use self::stream::Stream;
 
-use error::Reason::*;
 use frame::{StreamId, StreamIdOverflow};
 use proto::*;