Better output for Tagged Variants with non-inline record

该 issue 提供了一个递归 tagged-variant 示例，以及当前和期望的 JavaScript 输出；首先复现该示例，并跟踪生成 tagged-variant 记录的编译器路径。确定适用的 tag/字段名和单记录约束，然后验证该示例生成的是不带 _0 包装器的扁平字段。

Inline records in Variant provide good JS output, which is especially useful for bindings. When combined with tagged variants, they also provide a better DX.

However, it’s not always possible to inline records (or spread them in the case of recursive types), so I’m wondering if it would be possible to improve the JS output in the case of tagged variants.

One possible constraint would be to verify that the tag does not conflict with any of the record fields. We could also keep the same constraint as inline records: only one record.

@tag("nodeType")
type rec node =
  | @as(1) Element(element)
  | @as(3) Text(text)
  | @as(9) Document(document)
and element = {
  childNodes: array<node>,
  ownerDocument: null<document>,
}
and text = {
  nextElementSibling: null<element>,
}
and document = {
  children: array<element>,
}

let anElement = Element({
  childNodes: [Text({nextElementSibling: Null.null})],
  ownerDocument: Null.null,
})

Current output:

let anElement = {
  nodeType: 1,
  _0: {
    childNodes: [{
        nodeType: 3,
        _0: {
          nextElementSibling: null
        }
      }],
    ownerDocument: null
  }
};

Wanted output:

let anElement = {
  nodeType: 1,
  childNodes: [{
    nodeType: 3,
    nextElementSibling: null
  }],
  ownerDocument: null
};