IpRange wildcard prefix expansion uses ::/1 for IPv6

[immanuwell]

IpRange::Wildcard.iter_prefixes() looks like it under-covers IPv6.

Wildcard.contains() matches all IP addresses, but iter_prefixes() currently returns ::/1 for IPv6. For wildcard expansion, I think this should be ::/0, otherwise only half the IPv6 space is covered.

Repro:

let pfxs = IpRange::Wildcard.iter_prefixes().collect::<Vec<_>>();
assert_eq!(pfxs[1], "::/0".parse()?);

I have a tiny fix plus regression test here:
main...immanuwell:fix-iprange-wildcard-ipv6

I could not open a PR because the repo currently limits PRs to collaborators. Would you be open to taking this patch?

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