a 1.5% speedup in the JavaScript iBench

        * kjs/ustring.h: Add a hash field to UString::Rep.
        * kjs/ustring.cpp:
        (UString::Rep::create): Set hash to uninitialized value.
        (UString::Rep::destroy): Do the deleting in her, and call Identifier if needed.
        (UString::Rep::computeHash): Added.
        (UString::append): Set hash to 0 when modifying the string in place.
        (UString::operator=): Ditto.

        * kjs/property_map.cpp: Use the hash from UString.

        * kjs/identifier.h: Added aboutToDestroyUStringRep.
        * kjs/identifier.cpp: (Identifier::aboutToDestroyUStringRep): Added.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@2769 268f45cc-cd09-0410-ab3c-d52691b4dbfc