Function find_exprs_in_expr 

fn find_exprs_in_expr<F>(expr: &Expr, test_fn: &F) -> Vec<Expr>
where
    F: Fn(&Expr) -> bool,

Search an Expr, and all of its nested Expr’s, for any that pass the provided test. The returned Expr’s are deduplicated and returned in order of appearance (depth first). todo: document about that columns may refer to a lambda parameter?