Unicode section needs to be updated

Several issues I just noticed:

• open doesn't even list the :external-format keyword and other keywords mentioned in the description.
• Methods for strings needs updating because some of these functions are actually in the unicode package.
  • Update string-downcase and friends
  • Update string= and friends
  • Update string-equal and friends
  • Update string-trim and friends
• Add some missing documentation
  • WITH-STRING-CODEPOINT-ITERATOR
  • WITH-STRING-GLYPH-ITERATOR
  • New loop keywords being the codepoint and being the glyph
• Reader supports #\u+nnnn where nnnn is larger than 16 bits. While not really quite legal, it is immensely useful.