Reduced initial VM_STACK page size of Generators to 4 KB (256 slots)

Reduced initial VM_STACK page size of Generators to 4 KB (256 slots)
Generators all have their own vm_stack, but usually don't go really deep.
In most cases even most of the 4 KB will remain unused (in my tests average is rather 1 KB), but this was tested a straightforward implementation without a lot of abstraction layers.
That way I'm using a more conservative 4 KB stack size which really should be enough for, I think, at least 90% of the cases.
This was necessary to not immediately run out of memory with 95%+ usused vm_stack space: If you have a few thousands of Generators active, your application quickly hits memory_limit with a stack page size of 256 KB...
In addition, it's also a bit faster (70% less instructions for zend_vm_stack_new_page) due to emalloc() not having to allocate a whole new segment (segment size 256 KB). Also no mmap()/malloc() necessary.