Rollback dns_parser.rs from mdns-parser subcrate to mdns-sd crate

[keepsimple1]

This is rolling back the changes of PR #284 . The main reasons are:

• Increased complexity of this repo. I felt it's more difficult to maintain mdns-sd now.
• According to crates.io, it seems not many users of the extracted sub crate mdns-parser.

Overall, I felt it was not a good trade-off at this point. If we were to extract the parser in the future, it's probably better / cleaner to move it to its own repo.