refactor: use camino's UTF-8 paths everywhere

[andrius-puksta-sensmetry]

Also use camino-tempfile instead of tempfile to get UTF-8 paths for tempfiles.

Closes #136.

[victor-linroth-sensmetry]

Question: Should wrapfs also be converted to camino paths/pathbufs? If there is nothing hindering us from doing this it seems like it could make things a bit more concise and uniform.

[andrius-puksta-sensmetry]

Question: Should wrapfs also be converted to camino paths/pathbufs? If there is nothing hindering us from doing this it seems like it could make things a bit more concise and uniform.

I left some of them using std Path because we use std paths returned from dirs crate to call wrapfs. On second thought, though, since this is the only use of std paths remaining, I'll just call the std API directly to avoid a verbose conversion.

[victor-linroth-sensmetry]

I left some of them using std Path because we use std paths returned from dirs crate to call wrapfs. On second thought, though, since this is the only use of std paths remaining, I'll just call the std API directly to avoid a verbose conversion.

Long term I'm considering removing the dirs dependency anyway, since we probably want more fine grained control over where to load config files, but there's no urgency at the moment.