Very experimental use of vctrs

[hadley]

Just a 15 minute experiment. I have no recollection of how the facet data structure works, so this is just banging rocks together.

Fixes #4175

[hadley]

• When implementing this for real, may want to do it in such away that the non-vctrs code lives on so we can give the users the ability to switch back to the old version if desired
• Need to fundamentally rethink how facet data matching works using vctrs primiitves

[thomasp85]

Closing in favour of #4868