Split into library and binary parts and implement fuzzer

[realchonk]

TODO:

• [x] use cargo workspaces
• [x] remove all dependencies on fuser from the library
• [x] remove anyhow dependency from library
• [ ] update ChangeLog once everything is done

[realchonk]

@asomers Feel free to review this now.

[realchonk]

@asomers do you have any objections besides the (incomplete) fuzzing? If not, I'd really like to finally merge this and improve the fuzzer later.