Print integers with lowercase when *print-case* is :downcase.

Could optimize SUB-OUTPUT-INTEGER and DIGIT-TO-CHAR if needed, but I'm
assuming printing of fixnums and bignums is not limited by the
conversion of each digit to a character.

 * src/code/print.lisp:
   * Print integers in lowercase if *print-case* is :downcase
   * Update a docstring.

 * src/i18n/locale/cmucl.pot:
   * Update

 * src/general-info/release-20f.txt:
   * New file with updated info.