Fix for Directory Ordering compare function.

[TheSchemm]

Fixes #60

This is caused by the way that Rust does case folding. Rust does full case folding where the MS-CFB spec states that simple case folding should be used.
Because of the full case folding:
ßQÑ52Ç4ÅÁÔÂFÛCWCÙÂNË5Q== becomes SSQÑ52Ç4ÅÁÔÂFÛCWCÙÂNË5Q== where the ß should remain unchanged when using simple case folding.

This imports the icu_casemap crate that can do the simple case folding. I also made it so that string compares don't allocate new strings.

[sftse]

Thanks, that clears things up.

[mdsteele]

Ah, that makes sense. Thanks for figuring this out.

[mdsteele]

Thanks, looks good to me; just need to make clippy happy. (Either by taking its suggestion, or by suppressing the warning for the sake of the TODO; either way seems fine to me.)