Ensure string encoding for CFStrings work with non-UTF-8 pathnames

[jakewilliami]

Unfortunately on macOS, to check for whether a path is a package or bundle, the MDItem* functions necessitate the use of CFStrings, and all of the joys that come with them. As I have had to implement these myself, they are probably not very robust.

In the interest of everyone who doesn't use the Latin alphabet, I should ensure the string interface with Apple's Core Foundation works robustly.

[jakewilliami]

Might be able to ask @ScottPJones for some advice here, as he likes Julia, C, and strings...

[ScottPJones]

Hi! I was away in Spain for a while, just getting back to working - I'd be happy to chat about Julia & strings 😄