StackOverflow error when computing hash code of circular objects

This code will stack overflow:

```pkl
local myself {
res = myself
}

foo: Mapping = new {
[myself] = 1
}
```

Start by running the provided Pkl snippet and tracing the object hash-code computation for the self-referential `myself` object. Determine the intended behavior for circular objects and verify that computing the mapping key no longer causes a stack overflow.

Written by the indexing model from the issue text.