Replace Column expression with GetItem(Identity, Field)

[gatesn]

I would also pull this out into a function that walks expression trees:

    /// Accumulate all field references from this expression and its children in the provided set
    fn collect_references<'a>(&'a self, _references: &mut HashSet<&'a Field>) {}

    /// Accumulate all field references from this expression and its children in a new set
    fn references(&self) -> HashSet<&Field> {
        let mut refs = HashSet::new();
        self.collect_references(&mut refs);
        refs
    }

It's defined as exactly as the set of X where GetItem(Identity, X) exists in the expression tree.

And call it field_references instead.