look through references to improve externally tagged enum generation

[lsh]

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>.

[ahl]

In that specific case, I think we could generate better code if we picked apart the referenced SignalRef type:

#[derive(Clone, Debug, Deserialize, Serialize)]
pub enum AlignValueVariant0ItemSubtype1Subtype1Subtype0 {
    Signal(String),
    Value(AlignValueVariant0ItemSubtype1Subtype1Subtype0Variant1Value),
    Field(Field),
    Range(AlignValueVariant0ItemSubtype1Subtype1Subtype0Variant3Range),
}

This uses the default serde "external" enum tagging.

The downside (such as it is) is that this would no longer use the SignalRef type. Perhaps this could be mitigated by

impl TryFrom<AlignValueVariant0ItemSubtype1Subtype1Subtype0> for SignalRef {
    type Error = &'static str;
    fn try_from(value: AlignValueVariant0ItemSubtype1Subtype1Subtype0) {
        match value {
            AlignValueVariant0ItemSubtype1Subtype1Subtype0::Signal(signal) => Ok(Self { signal }),
            _ => Err("nope!"),
        }
    }
}

What do you think? Internally I think we'd do something like annotating each variant's IR to say "this is actually a type that we picked apart so offer up a TryFrom for this case".

[ahl]

The vega spec also seems to have many duplicated structures that might be nice to consolidate, though that would require us to figure out an appropriate name for the type. The current naming is, effectively, path dependent, and while not the nicest at least is simple to construct.

[ahl]

This turns out to be quite a pain in the neck right now. It may need to wait for #195 (in particular see "Separate mutable and immutable structures")

[ahl]

Another idea:

#[derive(Clone, Debug, Deserialize, Serialize)]
pub enum AlignValueVariant0ItemSubtype1Subtype1Subtype0 {
    Value(AlignValueVariant0ItemSubtype1Subtype1Subtype0Variant1Value),
    Field(Field),
    Range(AlignValueVariant0ItemSubtype1Subtype1Subtype0Variant3Range),
    #[untagged]
    SignalRef(SignalRef),
}

per serde-rs/serde#2403