PartiQL Shape `union` doesn't unify

[jpschorr]

After #485 PartiqlShapeBuilder.any_of:

    pub fn any_of<I>(&self, types: I) -> PartiqlShape
    where
        I: IntoIterator<Item = PartiqlShape>,
    {
        let any_of = AnyOf::from_iter(types);
        match any_of.types.len() {
            0 => type_dynamic!(),
            1 => {
                let AnyOf { types } = any_of;
                types.into_iter().next().unwrap()
            }
            // TODO figure out what does it mean for a Union to be nullable or not
            _ => PartiqlShape::AnyOf(any_of),
        }
    }

The AnyOf::from_iter(types) uses an IndexSet internally to deduplicate types, thus the match on any_of.types.len() could "flatten" AnyOfs that had duplicates. With the addition of IDs, this deduplication no longer happens...

Originally posted by @jpschorr in #485 (review)