Ask for an OsStr when converting between MdPath and RsPath.
Ask for an OsStr when converting between MdPath and RsPath.

Previously, `std::Path::Component` only implemented `AsRef<OsStr>`, so that's
the only thing you could get when calling `.as_ref()`. As of Rust 1.24, it also
implements `AsRef<Path>`, so we have to be more explicit.

pnkfelix/tango