The client Request now uses the same system as a server Response to track
the write status of headers, and the API has been updated accordingly.
Additionally, the Request constructors have been moved onto the Request object
instead of being top-level hyper functions, as this better namespaces the
client and Server.
This introduces a new Trait, NetworkStream, which abstracts over
the functionality provided by TcpStream so that it can be easily
mocked and extended in testing and hyper can be used for
other connection sources.
