More fast enumeration

[David Chisnall]

This patch fixes a bug in the last one (I forgot to set the items  
pointer) and adds enumeration support to GSArray and GSMutableArray.

GSArray just returns a pointer to its contents directly.   
GSMutableArray has a _version ivar added.  This is incremented every  
time the collection is mutated.  This allows the caller to check if  
the collection has changed between invocations and throw an exception  
if it has (this is done by calling objc_collectionMutation() which is  
not yet implemented on the GNU runtime).

David

fastenumeration2.diff
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