Consider using vctrs for underlying greta array / unknowns classes

In exploring print methods in #645, it might be worthwhile creating official vctrs classes.

These will hopefully help better manage a few key things

- Print methods (#333 #480 )
- Use in data frames (#625)
- `[`, `[<-`, `c`, `dim` methods for greta arrays

One thing I'm not clear on is how this works when the fundamental data type is a matrix, not a data frame, which is what `vctrs` seems to be focussed around.

It's worth a cursory explore, but it could also end up making things more complicated