pigpen/char.scm: New module

* pigpen/char.scm: New module.
* pigpen/encoder.scm: Use it.
char.scm (char->pigpen): Rename to `char->pigpen-char'

* char.scm (char->pigpen): Rename to `char->pigpen-char'.  All callers
  updated.
* pigpen/encoder.scm (string->pigpen-list): Update.
char.scm (pigpen-char->char): New procedure
char.scm (pigpen-char->string): New procedure