Use new read/write file method hooks to implement transparent support

Use new read/write file method hooks to implement transparent support
for compressed files.  Now Edwin will automatically uncompress a
compressed file when it is read in, and recompress it when it is
written back out.  Backups are handled in the usual fashion, except
that the backup file name ends in "~.Z" instead of ".Z~"; this allows
compress and uncompress to work normally with backup files.