Remove Sort from Expr

[findepi]

There are other Expr variants that only make sense in certain contexts as well. We can't have an Expr::Sort in a projection, for example.

That's a good point and good example. To me, in ORDER BY <expr> [ASC/DESC] [NULLS FIRST/LAST], the <expr> part is an expression (any expression), and the other attributes (asc/desc, nulls first/last) are attributes of the sorting. They don't have to be modeled as "an expression".

I think Sort would be an easier thing to remove / fix -- Expr::Sort as an expression is also bad as it means the signatures of fn order_by(...) are in terms of Expr, meaning the compiler can't ensure you are actually passing Expr::Sort when needed

Originally posted by @alamb in #1468 (comment)

[findepi]

take