Calculates the _compat value which is used for call-compatibility checks for simple calls. Whenver any of the dependent values are updated this must be called again. The dependent values include: _nparams - this is readonly, and never requies an update _defaults - the user can mutate this (func_defaults) and that forces an update expand dict/list - based on nparams and flags, both read-only Bits are allocated as: 00003fff - Normal argument count 0fffb000 - Default count 10000000 - unused 20000000 - expand list 40000000 - expand dict 80000000 - unused Enforce recursion is added at runtime.