bound on glob functions should be expanded to AsRef<Path>

[JamieH01]

currently, glob functions only take &str. changing this to impl AsRef<str> would be a trivial and nonbreaking change that makes usage easier and is almost always prefered for functions that take &strs. Additionally, AsRef<Path> would be much more useful when working with the fs module, but is a breaking change and may need more work as most string manipulation functions arent available for Path.