fix function_casts_as_integer warning

Comparing addresses of function pointers is brittle.

Functions may be duplicated resulting in function pointers to the same function to compare !=.
Functions may be merged/de-duplicated resulting in function pointers to different function to compare ==.

The later shouldn't be relevant here as the function differ in behavior, but mentioning it for completeness.

src/machine/machine_state_impl.rs

@@ -64,8 +64,7 @@ impl MachineState {
             cc: 0,
             global_clock: 0,
             dynamic_mode: FirstOrNext::First,
-            unify_fn: MachineState::unify,
-            bind_fn: MachineState::bind,
+            occurs_check: &Nsto,
             run_cleaners_fn: |_| false,
         }
     }
@@ -951,7 +950,7 @@ impl MachineState {
             }
         };
 
-        (self.bind_fn)(self, r, f_a);
+        self.occurs_check.bind(self, r, f_a);
         Ok(())
     }