Reviewed by Geoff Garen.

        - fix https://bugs.webkit.org/show_bug.cgi?id=20673
          single-character strings are churning in the Identifier table

        1.007x as fast on SunSpider overall
        1.167x as fast on SunSpider string/fasta

        * JavaScriptCore.exp: Updated.
        * kjs/SmallStrings.cpp:
        (KJS::SmallStrings::singleCharacterStringRep): Added.
        * kjs/SmallStrings.h: Added singleCharacterStringRep for clients that
        need just a UString, not a JSString.
        * kjs/identifier.cpp:
        (KJS::Identifier::add): Added special cases for single character strings
        so that the UString::Rep that ends up in the identifier table is the one
        from the single-character string optimization; otherwise we end up having
        to look it up in the identifier table over and over again.
        (KJS::Identifier::addSlowCase): Ditto.
        (KJS::Identifier::checkSameIdentifierTable): Made this function an empty
        inline in release builds so that callers don't have to put #ifndef NDEBUG
        at each call site.
        * kjs/identifier.h:
        (KJS::Identifier::add): Removed #ifndef NDEBUG around the calls to
        checkSameIdentifierTable.
        (KJS::Identifier::checkSameIdentifierTable): Added. Empty inline version
        for NDEBUG builds.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@36124 268f45cc-cd09-0410-ab3c-d52691b4dbfc