COAttributedString: rewrite substringItemGraphWithRange: to be faster (avoid copying the entire string). Helps with Typewriter slowness

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10588 7b3f36aa-e6db-49c1-8b4c-66eac1997e64