did:dht refactor step 1

[mistermoe]

Summary

this PR includes the first of 3 steps to refactor did:dht creation and resolution.

Overall plan is:

1. refactor DNS packet package
2. write bridge classes to convert DID Document resources <-> TXT Records
3. refactor actual did:dht Create and Resolve

Warning

you'll notice that a dns and dns_packet dir exists now. we've kept dns is the refactor. dns_packet is what's currently being used for did:dht creation and resolution. We'll be deleting dns_packet as part of the final PR.

Changes

• Refactored dns packet related stuff in order to allow for programmatic creation of individual packets and records
• Added tests for dns packet encoding and decoding
• Put a pattern in place for encoding/decoding that removes the need for adding switch statements everywhere. this should streamline adding support for additional RData types (not a priority but eventually would be nice to separate dns related functionality out into its own open source lib)