This commit involves several breaking changes:
* `session_key` config param must be a 256-bit base64 encoded string.
* `FromRequest` is implemented for `Cookies`, not `Cookie`.
* Only a single `Cookies` instance can be retrieved at a time.
* `Config::take_session_key` returns a `Vec<u8>`.
* `Into<Header>` is implemented for `&Cookie`, not `Cookie`.