`ScalarValue` should have an `ArrayRef` variant

[connortsui20]

#6523 merged vortex-scalar into vortex-array.

This now gives us the opportunity to add an ArrayRef variant to ScalarValue, which could potentially replace the List variant which holds a Vec<Option<ScalarValue>> right now (which is extremely heavyweight). You can imagine for an extension type like a Tensor that scalars would instantly become a bottleneck for any compute operations like matrix multiplication.

ScalarValue is technically public right now, but since it used to be completely opaque (until #6309) it should be fine to make this breaking change.