Doesn't support polymorphic variants

```reason
type state = [`Pending | `Success | `Error ];
```
Throw an error: "Unexpected core type, cannot convert"

Hoping can support some custom serialize function to override the default to string function.

Start by reproducing the polymorphic variant example from the issue and inspect the ppx_autoserialize entry point that raises "Unexpected core type, cannot convert." Determine how a custom serialization function should override the default string conversion; done means the example serializes successfully with that override instead of raising the error.

Written by the indexing model from the issue text.