Faster columnAtIndex:

I noticed columnAtIndex: taking around 50% of the time for a large fetch.

This is a little patch that makes that time effectively just go away.  It remembers the pointers to the keys that were used and does a pointer comparison first, falling back to the full computation if no pointer matches (and updating the pointer table in that case).

Would break if the client (a) used mutable strings to access the columns and (b) changed the content of those mutable strings during the course of evaluating the result set.