Allow tuples in untagged variants

[tsnobip]

Using tuples is a nice way to model heterogeneous arrays in rescript, but so far they're not allowed in unboxed/untagged variants, even though they're compiled to regular arrays in js, and arrays are allowed in untagged variants.

It'd be cool to add tuples to the allowed types in untagged variants.

See forum post for a use case.

We'd basically allow tuples and we'd check that only one variant case can be of type array or tuple.

[glennsl]

Related: #6277

[tsnobip]

Related: #6277

this is interesting, because indeed, we could solve the same issue by allowing objects to be compiled to arrays (it's likely much more work though), something like:

type t =
  | Foo({@as(0) foo: int, @as(1) bar: string})