Improve String handling in SIP crate

[t4lz]

As pointed out in #2195 (review), we incorrectly use Strings in the SIP crate in places where an OsString or a Vec<u8> would be appropriate.
I think if we do everything correctly, no error handling should be needed for string conversions, and also nothing like to_string_lossy.

There isn't any known issue caused by this yet, but it might be possible to construct an example where this incorrect handling would lead to SIP-sidestepping not working, maybe with some special characters or encoding in file paths or script shebangs.

[gememma]

Is there any reason we would need to use OsString instead of keeping everything as PathBuf? They do things like handle trailing slashes which is currently done manually

[aviramha]

Yes, the SIP crate patches/modifies OS-level strings, so to match layout/compatibility we have to use it too.

[gememma]

Is PathBuf not just a wrapper around OsString? Keeping everything as Path is still using the underlying OsString but with the guarantee that it's always producing a valid path

[aviramha]

Ah I didn't know that!