Whilst looking to implement #141 I couldn't work out a way to compare ScalarValue types.

There is, however, GroupByScalar which seems pretty compelling:

• It lacks an internal Option, allowing the caller to handle null-ordering how it deems fit
• It uses OrderedFloat, establishing a standards compliant ordering of floats
• It only provides the value variants that make sense to compare
• The plumbing to convert from a ScalarValue already exists

It therefore seems to be what I'm looking for, however, it doesn't currently have a PartialOrd or Ord implementation and is crate-local...

Based on this I have a couple of questions:

• Is there some other mechanism I should use to compare fields across record batches (at potentially different row offsets)
• Can we add #[derive(PartialOrd, Ord)] to GroupByScalar - note this would establish an ordering based on the order of enum variants between unrelated variants
• Can we expose GroupByScalar publicly, potentially renamed to something to reflect its more generic utility, perhaps ComparableScalar or something