This enables users to associate state with the callback without
relying on globals.

Also:
  * Refactor the tests for improved readability and extensibility, and
    to cover the new state object.
  * Drop the pointer from the `PyContext_WatchCallback` typedef.  This
    de-obfuscates the fact that pointers are involved, and makes it
    possible to forward-declare functions to improve readability:

        static PyContext_WatchCallback my_callback;

        int
        my_callback(PyObject *cbarg, PyContextEvent event, PyObject *obj)
        {
            ...
        }

… object