Clean up the dns_compress API

Merged Tony Finch requested to merge fanf-compression-relic into main

I was misled by doc/design/compression into thinking that might be a good place to do some rbt hacking, but no! I was misled!

It soon became clear that although the design documentation had not matched the code since December 2000, there was still a lot of weird complexity in the dns_compress API designed to cope with multiple compression methods, even though the IETF gave up on that idea over 20 years ago.

So, it's a good target for a little bit of modernization. And this patch set also removes a lot more than 500 lines of code and docs.

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