frame::rows() and expressions

Expressions mostly... work? frame::rows() works?

frame::rows() seems to mostly work now. It can't work with column names
unfortunately - only indices. But there might be a way of salvaging this
- although it would still mean uframe and useries would have to be used.
But for now, it's possible to craft an expression that filters on
columns based on arbitrary types that have standard c++ operator
support.