Add FFI type LocaleStatics and include casemap-relevant locales on it

[Manishearth]

Fixes #7603

[robertbastian]

I don't like the design of having a type that is only used to spawn references to another type.

These &'statics solve a problem for casemap, they should live on the casemapper. If another type also needs locale_nl() in the future, we can also add it on that type – a method that returns a pointer is very cheap.

[Manishearth]

These &'statics solve a problem for casemap, they should live on the casemapper. If another type also needs locale_nl() in the future, we can also add it on that type – a method that returns a pointer is very cheap.

This is a design I am open to, @sffc dispreferred it. Please see the discussion on #7642 (though that specific design may have been dispreferred when we discussed this in person)

The idea is that this type can add more locales over time if needed.

[robertbastian]

This is me joining the discussion

[Manishearth]

Yes, please continue the discussion there, because some of this has already been discussed a bit and I would not like to have to rehash things.