Make Tables.rowtable(result) as fast as Tables.rowtable(Tables.columntable(result))

This was raised here: https://github.com/invenia/LibPQ.jl/issues/259#issuecomment-1292675408

I think we should be able to use the Tables interface to either make it an equivalently fast operation, or just a straightforward dispatch to `Tables.rowtable(Tables.columntable(result))`.