Introduce common struct OxmPort for OxmInPort and OxmInPhysicalPort
Generalize marshal / unmarshal method for fixed size data
Return 0, err when error occurred in marshalFixedSizeData
Rearrange source file: marshalFixedSizeData() unmarshalFixedSizeData() moved to util.go
Fix build error
Define types for Value and Mask in OXM structures
Add comments to explain effective lower bits