Pickling nanobind functions (nb_func)

[awni]

Just curious on how possible to have support for this or to extend nb_func to allow it to be pickled?

This comes up in MLX since compiled functions are nb_func. And sometimes it's convenient for a Python class to hold a function as an attribute. We can work around it by wrapping stuff in user defined Python classes.. but it's inconvenient and confusing for users who do not know about this.