Single Stack for all generators

Hi, Nikita, please review the patch https://gist.github.com/dstogov/06116f1610f45f8152ine3a9927c6c243ac It's the next attempt to use the single stack for all generators. Now I don't see any problems or BC breaks. In case "yield" is used as an expression in context of function call. e.g. var_dump(yield), engine backups and then restores active call frame chain. Bob tested the patch on application that intensively uses generators, and reported: 15% less L2 cache loads 20% less cycle stalls due to memory subsystem 3% more CPU_CLOCK_UNHALTED due to additional freeze/restore. I'm going to commit this patch tomorrow. Thanks. Dmitry.