[Feature Request] Warn when unserializable values are returned from a view function

# 🚀 Feature Request

The following code returns invariant violation error, because the return value (anonymous closure) is not serializable.

```move
#[view]
public fun dummy_view(): || {
    || {}
}
```

Instead, a compiler warning should be given if we can detect view functions that return unserializable values.
