strings are strings after all

[dimbleby]

A possible approach to restoring sanity on the interface and returning strings as &str.

Per c-ares/c-ares#769

• very old versions of c-ares made no checks that strings were valid ascii
• since 1.17.2, c-ares has validated that hostnames are valid strings
• since version as-yet-unreleased, c-ares has validated that all of the other strings that it returns are valid strings

it's clearly desirable to return strings as &str on the rust bindings, but how to deal with the possibility that we are using an old c-ares that has not checked that this is true?

the approach here is: make use of rust's checking, and just fall over - unwrap() - if we receive a string that is not valid. The justification for that being something like:

• this was, if you squint a bit, always how the c-ares API was supposed to work
• so we view the 1.17.2 and as-yet-unreleased validation as bug fixes
• in the unlikely event that anyone is making queries that return invalid strings, and they fall over, our answer is "yes that was a c-ares bug which you can resolve by updating to the latest version of that project"

obvs that last one only really works when as-yet-unreleased is in fact released

well I'm not going to release this immediately so I guess this pull request can sit here for a bit and I will see whether I still buy that reasoning later...