Add support for recursive enums

[data-sync-user]

In mozilla/uniffi-rs#381 we added support for enums with associated data, but it doesn't properly support recursive enums, like the classic "tree" example where the enum variants are Tree::Leaf and Tree::Branch(left: Tree, right: Tree). Some notes on what we'd need in order to support such:

• In the rust code, we'll need to be prepared to Box the values in the recursive fields.
• In the swift binding, we'll need to use indirect keyword to support recursive enums.

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