test that inferring the function pointer type for a C function is enabled in safe type inference mode

fix ticket #644: infer type of C-API optimised methods of builtin types as Python object instead of C function

fix decorator lookup by avoiding (re-)assignments to the function/class name before their execution (ticket #593)

re-allow Python values in ExprStatNode and NameNode in nogil sections if they do not need cleanup (e.g. cached builtin names or constants)