external-formats: Add support for MacRoman character encoding.

  * This is rarely used these days, having been supplanted by the
use of UTF-8 with the adoption of OSX, but it is occasionally seen
"in the wild", or if someone has an interoperability concern with
antique macs.  I was rather surprised to encounter a CSV file
recently, with data for an event in May of 2013, that happened to
be MacRoman encoded, and then further surprised to find that SBCL
had no support for the encoding.

  * I have tested this interactively only, using OCTETS-TO-STRING
to verify that a sequence of the upper half of the 8-bit encoding
space was correctly mapped to unicode characters.