Reviewed by Maciej Stachowiak.

        Refactor named getter function signature to be in line with indexing getter signature
        https://bugs.webkit.org/show_bug.cgi?id=35563

        This removes the PropertySlot argument from getter functions, and makes them directly
        pass the slot base.  This makes the semantics for the functions match that of the
        indexing getters.

        On the down side, this means that we can no longer simply use a proxy function for
        JS getters, so we now add another marker value to indicate that a getter is present
        and branch accordingly.

        Against all rationality sunspider reports this as a perf win, but i suspect it's just noise.

        * API/JSCallbackObject.h:
        * API/JSCallbackObjectFunctions.h:
        (JSC::::staticValueGetter):
        (JSC::::staticFunctionGetter):
        (JSC::::callbackGetter):
        * JavaScriptCore.exp:
        * runtime/JSActivation.cpp:
       ...