encoder.scm (encode/ascii): Rename to `encode/unicode'

* pigpen/encoder.scm (encode/ascii): Rename to `encode/unicode'.  All
  callers updated.
  (encode): Update.
encoder.scm (encode/unicode): Rename to `encode'

* pigpen/encoder.scm (encode/unicode): Rename to `encode'.
  (encode): Remove.
string.scm (fill): New procedure

* pigpen/string.scm (fill): New procedure.
* utils/pigpen.in: Add `--fill' option.