Keep encoding information in `Txt` `RR`s

Keep encoding information in `Txt` `RR`s

Sadly `TXT` records can be encoded in many ways (as they're
chunked, and chunks can be any size), and are signed in the way in
which we receive them on the wire. Thus, we must keep the encoding
information we receive on the wire around in `Txt`s to ensure we
can validate their signatures.

Here we do so, creating a pile of new machinery to store `Txt` data
as a series of up-to-255-byte chunks.