Fix panics in ClientSubnet conversions

[djc]

#1906 (cc @mokeyish) introduced From conversions that called expect(), which caused errors in OSS-Fuzz. Replace these From impls with TryFrom instead, and upgrade to 2021 edition where TryFrom/TryInto became part of the prelude.

[mokeyish]

I found that too, but I follow Minimal Changes. So continue using From/Into.

[djc]

As far as I can tell the From implementations for the other EDNS types were actually infallible, unlike the new type you added. Ideally we would catch this during review, but writing code that is correct/robust to failures is more important than "minimal changes".

[bluejekyll]

Thanks, @djc, I missed the expect calls there.