Allow users to access IP/TCP/UDP headers

This will allow users to access packet metadata without needing to parse
the "parsedPacket" string, and will also allow them to access the packet
data present after the headers for further computation, because the
header addresses are now exposed.

The IPHeader, TCPHeader, UDPHeader structs have been moved outside
PacketFragmentArgs to allow definition of properties with the same name.
They have been converted from structs to class to make them into
reference types and therefore have the homonymic properties return null
if the corresponding header was not successfully parsed by
ParsePacket().

I am proposing this change because I need to parse DNS request/response
in packets, and it is almost impossible to do it with the current class
structure without duplicating the code of FIndIpHeader(), and I think
that it's much cleaner to expose the headers themselves rather than just
exposing FindIpHeader() and having the users duplicate the parsing
logic.

Author: lupino3