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`.
* The `unmanaged_state` lint emits a warning when a `State<T>` request
guard is used without an accompanying `manage` call for `T`.
* The `unmounted_route` lint emits a warning when a route declared via
a Rocket attribute is not mounted via a call to `mount`.
There is one known shortcoming of these lints at present: _any_ call to
`manage` or `mount` marks state/routes as managed/mounted. This can be
an issue when an application uses more than one `Rocket` instance, with
different calls to `mount` and `manage` in each. The lints should
perform their analyses on a per-instance basis.
