Pervasives.compare : different semantic with abstract value

[hhugo]

comparing 2 different Weak.t will succeed with js_of_ocaml where it should raise an exception.
We should either fix the runtime or document the deviation.

[hhugo]

@vouillon, any thoughts ?
I would fix the runtime and raise exceptions when needed.
I've had issues in the past, comparing React.signal with the default comparison. Weak.t inside React.signal were introducing cycles (comparing never stops)