Reevaluate if all struct elements should be pub

[JamesHuard]

Currently all descriptor fields are marked public. This allows maximum flexibility but does allow invalid configuration of descriptors. After we have some mileage using these in the field, we should re-evaluate if things like the crc field could be made private and constructors used to verify integrity at construction time, perhaps offering some way to hook in CRC hardware acceleration for particular use cases.