Unify PatternMessage and SelectMessage

[stasm]

The data model proposed in #393 defines Message as the following union type:

 type Message = PatternMessage | SelectMessage

 interface PatternMessage {
   type: 'message';
   declarations: Declaration[];
   pattern: Pattern;
 }

 interface SelectMessage {
   type: 'select';
   declarations: Declaration[];
   selectors: Expression[];
   variants: Variant[];
}

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Seems like it might be simpler to have a single message, which has a body attribute that's one of pattern or select, and then select would just have the selectors and variants fields? (In other words, declarations would be factored out.)

Originally posted by @catamorphism in #393 (comment)

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There's also the approach @stasm has suggested, i.e. expressing single-pattern messages as having an empty list for selectors, and a single entry in variants with a corrspondingly empty list of keys.

As I mentioned on the call, I'd much prefer landing something first and then iterating on it, rather than sorting out the details all in one go.

Originally posted by @eemeli in #393 (comment)

[aphillips]

Is this still relevant?

[catamorphism]

Is this still relevant?

Since I made the original suggestion: This would be a very minor refactoring of the data model spec, which would make the spec easier to read but doesn't have any impact from users. I would personally be fine with just closing the issue as wontfix, since we have plenty to do.

[aphillips]

Thanks. I'll close this.

One reason I asked if this were still relevant is that our syntax has changed in a different direction (simple vs. complex) since then.