Make RData and RRType the same thing so it becomes impossible to make
a mistake.

Tests needed:
 - encode and decode of each kind of RR
    - so far, have: txt, a, ns, mx, soa, cname, aaaa, srv.
    - that leaves: md, mf, mb, mg, mr, null, wks, ptr, hinfo, minfo
    - most of those are obsolete, so finding wild examples will be
      very difficult

 - encode and decode of several variants of packet, both sensible and not
    - misleading section length count - short, long
    - misleading label length - short, long
    - misleading rdata length - short, long
    - misleading txt record string length - short, long
    - looping domain-name (using compressed format)
    - compressed domain-name pointing into hyperspace
    - txt record with rdata filled with a list of empty byte-strings