[mc] Extend xbt_propositional_symbols

Add support for:

 * general callbacks with a `void*` argument (the `data` field);

 * as an optimisation, a simple pointer to a variable can be passed in
   the `data` field (this can be used in order to enable the MCer to
   fetch the data directly from the MCed memory without trigerring a
   MCed-side callback);

 * a callback function used to free the `data` field.