Storing guardPageSize in Fiber

[Johan Engelen]

Hi all,
   Currently, `guardPageSize` is not stored in core.thread.Fiber. 
It's only used during construction, so it's currently not 
necessary to store it. However, for Fiber stack usage tracking, I 
need it elsewhere too.
Is it agreeable to add an m_guardPageSize member to Fiber to 
store the information
(i.e. increasing Fiber's size a little). Or is there another way 
to recover that information later on?

Thanks,
   Johan