[squirrelfish] I am removing ExecState

I'm about 90% done getting rid of ExecState. Now ExecState and JSGlobalData are one and the same. I think it will be a speedup once I get the correctness issues resolved. This was inspired by Geoff, who suggested making ExecState be a call frame pointer. I think that's still a reasonable alternative; we can debate the pros and cons on Monday. The downside of ExecState being JSGlobalData is that you have to store the call frame pointer in a global as you make function calls. The upside is that you don't have to chase a pointer to get to global data. -- Darin