Stats in OpenFlow0x01_Stats

Makes stuff a lot cleaner. There are overlapping fieldnames between OpenFlow0x01_Core
and OpenFlow0x01_Stats, so you will typically do

    open OpenFlow0x01_Core
    module Stats = OpenFlow0x01_Stats

OpenFlow0x01 remains a big mess. But, it is not exposed to students. Defer
cleaning it up for later.

This closes #43