Speed up readtable

Adds a little complexity to the IO code, but I think it's worth it.

I'm seeing a 2x speed up on 25-row tables, a 3-3.5x speedup on 100K-row tables, and a 5x speedup for 25-row tables with many bytes per field.

Also, the hack of creating a tuple based of values of ParseOptions and dispatching on the tuple, apart from being ugly, may have added some (under 2%) overhead -- I'd appreciate any suggestions / tricks.