set_pin_async
Description:
public async bool set_pin_async (uint8[]? old_pin, size_t n_old_pin, uint8[]? new_pin, Cancellable? cancellable) throws Error
  
  Change the user's pin on this slot that this session is opened on.
This call will return immediately and completes asynchronously.
Parameters:
| this | Change the PIN for this session's slot. | 
| old_pin | the user's old PIN, or  | 
| n_old_pin | the length of the old PIN | 
| new_pin | the user's new PIN, or  | 
| cancellable | Optional cancellation object, or  | 
| callback | Called when the operation completes. | 
| n_new_pin | the length of the new PIN | 
| user_data | Data to pass to the callback. |