This way handlers don't need to do case matching on backend ID strings
anymore.
* Rename `backend` to `backend_id` where we have a backend ID
* Add `get` function and `Backends` enum to the `backend` module
* Add a depend on the `enum_dispatch` crate
* Add a `backend` module `Backend` trait and necessary abstract types
* Refactor handlers to use the back-end abstraction
* Directly serialize to URLs where necessary in Mixcloud back-end
* Require `serde` feature for the url crate
* The feed item limit defaults to the default page size (50) if not
provided
* Move caching from response to URL fetch results; add helper functions
* Add a helper function to set the paging query of an URL
* Modify paging so we don't retrieve more than the feed item limit
* Derserialize the paging information
* Parse each next URL; handle URL parse errors
* Use a default page size of 50; pass offset 0 to count by item index
* 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
* Prefix the feed handler path with '/feed'
* Mount both handlers under `/` now that they have a prefix in the path
* Provide the backend to the feed and download handler in their paths
* Use the ID as filename download handler, also add an extension
This might solve some issues with clients that don't get the file type
and size and don't know what to do?
Also, we're not longer involved in the download, yay! (For now.)
Paul van Tilburg