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.
Introduces two Phantom Types, Fresh and Streaming, which indicate the status
of a Response.
Response::start translates an Response<Fresh> into a
Response<Streaming> by writing the StatusCode and Headers.
Response<Fresh> allows modification of Headers and StatusCode, but does
not allow writing to the body. Response<Streaming> has the opposite privileges.
This introduces a bit of complexity to the example,
mainly the use of the try_continue! macro for dealing
with errors, but I think this is an appropriate trade-off
as users of this library are likely to be framework authors
instead of end users.
