cipher.scm (%ascii-mapping): Rename to `%unicode-mapping'

* pigpen/cipher.scm (%ascii-mapping): Rename to `%unicode-mapping'.
* pigpen/char.scm (list->pigpen-char, char->pigpen-char): Update.
cipher.scm (%ascii-mapping): New variable

* pigpen/cipher.scm (%ascii-mapping, %ascii-substitution-table): New
  variables.