[skrifa] Expose x-height and stem width (and more?) computation

It would be useful to be able to easily access _at least_ x-height, and ideally other things (stem width, etc) that are computed as part of autohinting.

Per IM this could be exposed as something like an `outline::topology` module.

For context, and potentially to inform API design, intended use isn't in direct text rendering path but might want to process many fonts at once. Doesn't have to be _fast_, though it would be nice if it doesn't result in excessive API hostility.