* Enable the `async` feature for the `cached` crate
* Make the types that we cache implement `Clone`
* Rename the argument of mixcloud::redirect_url` because of this issue:
https://github.com/jaemk/cached/issues/114
* Use the `thiserror` crate to make our own error type
* Implement Rocket's `Responder` type for the error type
* Adjust all methods to use the error type
* Small documentation tweaks