look through references to improve externally tagged `enum` generation

This is a possible solution to some of the gnarly Vega code generation. Several of the more complex types end up with many VariantN layers (like @ahl mentioned in #165).

For example we have:

#[derive(Clone, Debug, Deserialize, Serialize)]
#[serde(untagged)]
pub enum AlignValueVariant0ItemSubtype1Subtype1Subtype0 {
    Variant0(SignalRef),
    Variant1 {
        value: AlignValueVariant0ItemSubtype1Subtype1Subtype0Variant1Value,
    },
    Variant2 {
        field: Field,
    },
    Variant3 {
        range: AlignValueVariant0ItemSubtype1Subtype1Subtype0Variant3Range,
    },
}

It might be okay to replace instances of that enum with something like Box<dyn AsVegaAlign>.

No files or tests are named. Start by tracing externally tagged enum generation and references to the generated VariantN types, using the Vega example and issue #165 as context; determine whether replacing these enums with Box is viable and define the expected generated output before implementation.