Init nix flake, fix build, add clippy+rustfmt

[Ma27]

So long story short I had this package in my local nixpkgs overlay, but unfortunately using git dependencies with rustPlatform.buildRustPackage became rather painful after the switch to rustc 1.68. Anyways things escalated a bit, but I figured that some of the patches at least may be useful for you.

---

• https://github.com/FliegendeWurst/KIT-ILIAS-downloader/commit/0396dfc096c9547899b8e9da5b85549c505b0dcc simply fixes the compiler errors on master.
• The next two commits (05abc5bdad49763b7f593346437e51e006f61b0f & 3f326713ba440cf38661e7e1d6776a1cd3e5eeb0) make clippy & rustfmt happy.
• 4c90a6029a24bf933582d550059ffbd293c19135 adds a nix-based dev & build environment with the following features:
  • uses crane for building. Unlike traditional rust builds with Nix it separates dependencies and the project's code into two derivations, so it's even for developing quite useful. nix build builds this package, with nix run you can execute the program.
  • nix flake check can be used to run rustfmt/clippy.
  • A shell with all dev dependencies (cargo/rustc/clippy/etc) can be opened via nix develop. Alternatively, you can also use direnv to load all of that when cding into your local checkout.

My main motivation was to have a nice dev environment + convenient way to build this beautiful project and to rebase all changes from this repo into my fork for now. Since you're also using Nix(OS) I figured that not only the build fix, but also the flake stuff may be of interest for you, so I filed a PR for all of that. Just cherry-pick whatever you'd like if you want to :sweat_smile:

Note: I haven't updated CI and all of that since the current state is good-enough for me currently.

cc @FliegendeWurst

[FliegendeWurst]

Thanks for the changes and fixes! I haven't used Nix flakes before, but at a glance your flake.nix looks good. I will try it out later.