Changes

#Eliminate SP update. Inside basic blocks of JSOPs, we shouldn't need to keep a proper stack. Instead, we can teach the compiler to track which logical stack element is in which register and generate faster code.

#Fast closures. This is important for advanced web apps as well as Dromaeo and the V8 benchmarks. See [https://bugzilla.mozilla.org/show_bug.cgi?id=517164 bug 517164].