Add FontRef::from_face function.

[coevolutions]

This allows FontRefs to be already created from pre-parsed faces. This can be useful to avoid having to reparse a face if we already have one, e.g. if we want to do shaping with rustybuzz, we should already have a parsed face and parsing it a second time is wasteful.

[alexheretic]

This would expose ttf-parser api publicly which is something I deliberately avoid here for semver stability.

[coevolutions]

I see - I would like to avoid having to reparse the Face though if I already have it. Would you be okay with hiding it behind a feature flag?