1.0.32.21: compress most unibyte-external-format definitions

1.0.32.21: compress most unibyte-external-format definitions

All the unibyte-mapper-based external-formats had huge amounts of
cut-and-pasted code, differing only in names of functions.  This is,
oddly enough, a clear case for abstracting away the repeated code into a
macro.

In the process, convert them to the multibyte apparatus, which has support
for the nice restarts, and remove the too-simple unibyte
DEFINE-EXTERNAL-FORMAT (and EXTERNAL-FORMAT-DECODING-ERROR) which are now
unused.

Include a far-from-comprehensive set of tests, which are mostly for
iso-8859-x formats