Consider making root-collation CollatorBorrowed const-constructible from baked data

[hsivonen]

If CollatorBorrowed was const-constructible, it would be possible to have a CollatorBorrowed-typed mut static (without wrapping in Option or MaybeUninit) that could be overwritten with a dynamically-initialized CollatorBorrowed from main.

[Manishearth]

This seems fine to do, and typically how we do borrowed types.

Does this actually need to be discussed?

[Manishearth]

Stating explicitly: I'm in favor of const CollatorBorrowed::new_root()

[sffc]

It could also be named new_invariant given that in Segmenter we call it InvariantOptions.

[Manishearth]

We should do this; I don't think it needs to be discussed.