Improved multicast DNS support

• Add support for multicast DNS (RFC6762) in the trie. (#35 from Luke Dunstan)
  • mDNS doesn't use SOA nor delegation (RFC 6762 section 12), so some minor changes
    to Trie are required to handle this.
  • mDNS doesn't echo the questions in the response (RFC 6762 section 6), except
    in legacy mode, so a bool argument was added to Query.response_of_answer.
  • Query.answer still exists but now Query.answer_multiple is also available
    for answering multiple questions in one query to produce a single answer
    (RFC 6762 section 5.3). One caveat is that responses may exceed the maximum
    message length, but that is not really specific to mDNS. Also, in theory multiple
    questions might require multiple separate response messages in unusual cases,
    but that is complicated and the library does not deal with that yet.
  • Query.answer_multiple takes an optional function to allow the caller to control
    the cache-flush bit. This bit is only set for records that have been "confirmed
    as unique". Using a callback requires minimal changes here but puts the burden of
    maintaining uniqueness state elsewhere.
  • Query.answer_multiple takes an optional function to filter the answer, in order
    to support "known answer suppression" (RFC 6762 section 7.1). Again, using a callback
    requires minimal change to the core, but later on the mDNS-specific known answer
    suppression logic could move into the Query module if that turns out to be simpler.
  • A query for PTR returns additional records for SRV and TXT, to support efficient
    service discovery.
  • Trie.iter was added to support mDNS announcements.
• Switch to Bytes instead of String for eventual -safe-string support.
• Partially remove some error printing to stderr. (#36)

Unit tests were added for some of the changes above, including a test-only
dependency on pcap-format.