Sorting attributes with constructed types

Currently attempting to sort a relation by an attribute that has a constructed type throws an `AttributeNotSortableError`. This is really limiting, since even a simple `Maybe` type becomes unsortable as well.

I think we need a solution that behaves like `sortBy` which takes an `AtomFunction` or maybe something like a `ValueExtractionExpr` (which behaves like a `RelationalExpr`) and provides the value on which the sort should be done.

Any Ideas on this?