String compare #7

This library produces a Cow<str> from its functions, which may allocate a new string if there is "prepping" to do. But would there be an efficient way to directly compare two strings as if they were string-prepped without actually doing it?

I would think it would be something like:

1. Akip all leading spaces, then
2. Any white space characters get treated like a space character and any number of space characters match one or more spaces in the other string, then
3. Case-fold, character by character
4. etc.

The theory is that a stringprep comparison could be done without ever allocating a new string. This would be a great feature to have!