nurl as a library

[figsoda]

Exposing a Rust library would be useful for projects like nix-init

• Rework the traits to get rid of the ugly impl_fetcher macro
• Move highly specific things like group out of SimpleFetcher
• Expose the fetcher detection heuristics in main

[eveeifyeve]

It would still be good to keep the cli version as well, but I am looking into this further to see if I can get a start.

[figsoda]

the cli will still be the main way to use nurl. feel free to give suggestions on what the api should look like, but don't start actual work unless you are ok with me closing it