Seroval: Deserialization invokes attacker-chosen callables: AsyncIteratorFactoryInstance calls source.on(listener) without type check

### Summary When `fromJSON` rebuilt an async iterator, it called `on()` on the iterator's source without checking that the source was a seroval stream. A crafted payload could pick another value as the source, such as a function returned by an application plugin. The deserializer then called that value's `on` method. This is a variant of GHSA-mv8w-475r-vwqw. That fix covered the promise and stream control nodes but not the async iterator node. ### Impact An application that passes untrusted input to `fromJSON` with plugins that return callable values can have those callables invoked during deserialization. What this allows depends on what the plugin's values do. ### Affected versions - Versions up to 1.6.3 accept any value as the source. - 1.6.4 rejects the reported payload. - 1.6.5 to 1.6.7 check the source by node type, which a payload can get around by reusing a node id. - All versions up to 1.6.7 should be treated as affected. ### Patches Fixed in `seroval@1.6.8`. Streams and sequences are now internal class instances. The deserializer checks the source value with `instanceof`, so plain data and plugin values are rejected no matter which node id they use. ### Workarounds Avoid plugins whose `deserialize` returns functions or objects with an `on` method when decoding untrusted input. Upgrading to 1.6.8 is recommended.