Tracking issue: Implement all of UTS 39

[Manishearth]

Here are the different parts of UTS 39 we may wish to implement:

Needed for RFC 2457:

• is_allowed() for the general security profile
• Mixed script detection.
• A method for knowing what script(s) a string is in (Support obtaining Script_Extensions of a character #4)
• A method for checking if a string has mixed scripts
• Restriction level 4: We only need level 4, but implementing all of them would be nice
• We don't need to implement full mixed script confusables for rustc, we can write a more targeted heuristic

Unnecessary for Rust, but nice to have:

• identifier_type() to return the exact categorization (Support Identifier Type #7)
• IDN security profile
• Email security profile
• Mixed number detection
• Full Whole script and mixed script confusables
• Optional Detection Out of scope, needs CLDR