Add function to load all unicode data into memory.

This makes it easy to make an executable image that doesn't need
unidata.bin around.  (Should we do this for normal cores?  It seems to
add about 1 MB to the core size.)

code/unidata.lisp:
o Add LOAD-ALL-UNICODE-DATA to load all unicode data.
o Add UNICODE-DATA-LOADED-P to check that unicode data has been
  loaded.

code/print.lisp:
o If unicode data is loaded, don't check for existence of
  *unidata-path*, because we don't need it.

code/exports.lisp:
o Export LOAD-ALL-UNICODE-DATA.

general-info/release-20c.txt:
o Update info