Reviewed by Darin Adler.
        
        - optimize appending a number to a string
        https://bugs.webkit.org/show_bug.cgi?id=21203
        
        It's pretty common in real-world code (and on some of the v8
        benchmarks) to append a number to a string, so I made this one of
        the fast cases, and also added support to UString to do it
        directly without allocating a temporary UString.
        
        ~1% speedup on v8 benchmark.

        * VM/Machine.cpp:
        (JSC::jsAddSlowCase): Make this NEVER_INLINE because somehow otherwise
        the change is a regression.
        (JSC::jsAdd): Handle number + string special case.
        (JSC::Machine::cti_op_add): Integrate much of the logic of jsAdd to
        avoid exception check in the str + str, num + num and str + num cases.
        * kjs/ustring.cpp:
        (JSC::expandedSize): Make this a non-member function, since it needs to be 
        called in non-member functions but not outside this file.
        (JSC::expandCapacity): Ditto.
        (JSC::UString::expandCapacity): Call the non-member version. 
        (JSC::createRep): Helper to make a rep from a char*.
        (JSC::UString::UString): Use above helper.
        (JSC::concatenate): Guts of concatenating constructor for cases where first
        item is a UString::Rep, and second is a UChar* and length, or a char*.
        (JSC::UString::append): Implement for cases where first item is a UString::Rep,
        and second is an int or double. Sadly duplicates logic of UString::from(int)
        and UString::from(double).
        * kjs/ustring.h:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@37089 268f45cc-cd09-0410-ab3c-d52691b4dbfc