Allow non-utf8 filenames

[jyn514]

Filenames on Unix can be arbitrary bytes; on Windows they can be arbitrary UTF16 (and I think there's some strangeness around surrogate pairs).

error[E0277]: the trait bound `std::string::String: std::convert::From<std::path::PathBuf>` is not satisfied
   --> src/main.rs:108:27
    |
108 |     let file_id = file_db.add(opt.filename, buf);
    |                           ^^^ the trait `std::convert::From<std::path::PathBuf>` is not implemented for `std::string::String`
    |
    = help: the following implementations were found:
              <std::string::String as std::convert::From<&std::string::String>>
              <std::string::String as std::convert::From<&str>>
              <std::string::String as std::convert::From<rcc::data::LengthError>>
              <std::string::String as std::convert::From<std::borrow::Cow<'a, str>>>
              <std::string::String as std::convert::From<std::boxed::Box<str>>>
    = note: required because of the requirements on the impl of `std::convert::Into<std::string::String>` for `std::path::PathBuf`

[jyn514]

I think the proper thing to do would be storing a OsString instead of a String. I can make a PR if it's welcome :)

[brendanzab]

Yes, this makes sense! This was my mistake! 🤦‍♂