char.scm (<pigpen-char>): Add cipher name field

* pigpen/char.scm (<pigpen-char>): Add cipher name field.  All related
  procedures updated.
  (pigpen-char->string, pigpen-char->list, pigpen-char->char): Update.
  (make-pigpen-char): Take a cipher name as the first argument.  All
  callers updated.
  (char->pigpen-char, list->pigpen-char): Update.