Add quick tests for domain name and TXT rr
Update domain name and TXT string escape behaviour

Changes to domain name packing and unpacking:
* Escape dot, backslash, brackets, double-quote, semi-colon and space
* Tab, line feed and carriage return become \t, \n and \r

Changes to TXT string packing and unpacking:
* Escape backslash and double-quote
* Tab, line feed and carriage return become \t, \n and \r
* Other unprintables to \DDD

Stringers do the equivalent of putting domain names and TXT strings
to the wire and back.

There is some duplication of logic. I found performance suffered when
I broke the logic out into smaller functions. I think this may have
been due to functions not being inlined for various reasons.