Expression enum and Root variant

[mhk197]

Makes Root an Expression variant instead of a ScalarFnVTable implementation.

This is groundwork for lambdas and variables — those are also nodes whose dtype comes from the
scope rather than from their children, and they need the same treatment.

Root is not a scalar function. Its dtype comes from the scope rather than from its children (it has no children) and it is not executable.

This PR makes Expression an enum instead of a struct. Root becomes a variant, and upcoming Lambda and Variable become variants as well.

pub enum Expression {
    Scalar { scalar_fn: ScalarFnRef, children: Arc<Vec<Expression>> },
    Root,
}