OsStr, WTF8, as_bytes, and to_string_unchecked

[kornel]

Related problem that would also benefit from exposing OsStr bytes on Windows:

PathBuf to CString

Conversion from a Path to a C-compatible string is not easy in libstd. Because the conversion is not well-defined on Windows, access to bytes is hidden behind cfg(unix). However, that means that in user code the correct conversion code for Unix also has to be behind cfg(unix), and requires writing a fallback implementation. That is a lot to ask for such a minor detail, so people end up writing CString::new(path.to_str().unwrap()) which is the worst case of all. I suggest adding TryFrom which …