Add helpers for converting LanguageIdentifier to LocaleFamily

[sffc]

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Part of #4629

[Manishearth]

I don't strongly feel like we need these APIs but I'm not opposed to adding them, defer to @robertbastian

[sffc]

I don't strongly feel like we need these APIs but I'm not opposed to adding them, defer to @robertbastian

Exactly why I made this its own pull request, so we can decide one way or another

[robertbastian]

I don't really see the need for these, and I'm also not a fan of the auto, and especially auto_iter names. Meh

[sffc]

This is mostly obsolete. Might take another look before 2.0