- In the future, we want to return some summary information about a
    parsed packet sequence after the packet sequence is fully parsed.
    Currently, PacketParser::next() and PacketParser::recurse()
    consume the PacketParser and return None on EOF.  Thus, even if
    the summary information were stored in the PacketParser, it
    becomes inaccessible on EOF.

  - This change introduces a new type, PacketParserResult, that
    contains either a PacketParser or a PacketParserEOF.
    PacketParserEOF is returned on EOF instead of None.  Since it is a
    struct, it can hold only any information that we want to return to
    the caller.