resolver

[hannesm]

another refactoring which i'll review tomorrow, fix the missing bits and pieces, and merge.

major changes include:

• Udns.Packet.t is now a record, type res is gone
• there's Packet.create (...) -> t
• and Packet.encode : ... -> t -> buffer
• the distinction between request and reply is now on the type level (a polyvar), type data is request or reply

puts recursive resolver back into business (still std iterative algorithm, not fancy qname minimisation), also revives some resolver tests.

[hannesm]

won't merge this, pushed to master directly...