stats: add sortiness score

[jqnatividad]

When computing sort_order of a column, we can maintain a "sortiness"¹ score.

Right now, sort_order has three states - Ascending, Descending and Unsorted.

sortiness will allow us to assess the general sort order of an unsorted column.

The score will range from -1.0 to 1.0.

-1.0 is equivalent to Descending. 1.0 is Ascending. 0.0 indicates that the column only has one value.

When sortiness is 0.0, we can even short-circuit and skip cardinality computation.

Footnotes

1. Inspired by Stephen Colbert's Truthiness 😉 ↩