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.
