Fast enumeration

Hi Everyone,

I've got this patch in my local branch for fast enumeration on the GNU runtime, and I've tested it with GNUstep. The GNU Objective-C runtime doesn't provide this function, but other libraries do. Since it is exactly the same as the Apple runtime implementation, I wonder if it would be better for CGObjC.cpp to just call this directly. It's technically runtime-specific, but I doubt anyone is going to want to implement exactly the same function and give it a different name just to be different, and if they do then they can add this in their runtime and everyone's happy:

void (*objc_enumerationMutation)(id) = my_really_stupid_function_name;


Patch applied and checked in.

- fariborz