Part of effort to remove `Expr::Sort`.

Part of effort to remove `Expr::Sort`.

Take `expr::Sort` in `LogicalPlanBuilder.sort`.
Accept any `Expr` in new function, `LogicalPlanBuilder.sort_by` which
apply default sort ordering.

Part of effort to remove `Expr::Sort`.

Part of effort to remove `Expr::Sort`.

Part of effort to remove `Expr::Sort`.

Remove sort as an expression, i.e. remove `Expr::Sort` from `Expr` enum.
Use `expr::Sort` directly when sorting.

The sort expression was used in context of ordering (sort, topk, create
table, file sorting).  Those places  require their sort expression to be
of type Sort anyway and no other expression was allowed, so this change
improves static typing.  Sort as an expression was illegal in other
contexts.

this commit is longer than advised in the review comment, but after
squashing the diff will be smaller