This removes the cookie crate, since it has an optional dependency on
openssl, which can cause massive breakage if toggled on. Instead, the
`Cookie` and `SetCookie` headers now just use a `String`. Anyone can
create any typed header, so it is easy to plug in different
implementations.
BREAKING CHANGE: The `Cookie` and `SetCookie` headers no longer use the
cookie crate. New headers can be written for any header, or the ones
provided in hyper can be accessed as strings.