feat(users/Profpatsch/whatcd-resolver): add a simple torrent parser

The `bencode` library is anything but production-grade, but it’s
enough to parse the torrent files generated by Gazelle lol.

This should help with … I haven’t really figured out yet what it helps
with I guess. But it was fun. I like the `Parse` abstraction very
much. It can parse XML and it can parse Bencode. Good.

Change-Id: If1331de423eab3e91ce9db6e2a7eb84da51b18a2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/13211
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>

users/Profpatsch/shell.nix

@@ -14,6 +14,7 @@ pkgs.mkShell {
       h.async
       h.aeson-better-errors
       h.blaze-html
+      h.bencode
       h.conduit-extra
       h.error
       h.monad-logger