Packet: Add derived class FunctionalPacket to enable partial functional reads

Packet: Add derived class FunctionalPacket to enable partial functional reads

This adds the derived class FunctionalPacket to fix a long standing
deficiency in the Packet class where it was unable to handle finding data to
partially satisfy a functional access.  Made this a derived class as
functional accesses are used only in certain contexts and to not add any
additional overhead to the existing Packet class.