Restore `GeneralDnsNameRef`, use in `GeneralName::DnsName`.

[cpu]

In #42 we adopted some upstream work that included a new GeneralDnsNameRef enum that had a DnsName variant for a standard DnsNameRef (no wildcards, can be used for subject matching) and a Wildcard variant for a new WildcardDnsNameRef type (wildcards supported, only used in the context of a presented DNS name). The original idea that prompted development of this enum is that it would support changing the GeneralName::DnsName enum variant from holding untrusted::Input to holding GeneralDnsNameRef. This allows more specificity while still maintaining a strong separation between the appropriate uses of the two types of name refs (see https://github.com/briansmith/webpki/issues/66).

We never fully implemented this idea after landing #42 and using it only in the context of iterating certificate SAN dNSNames for informational purposes. We considered exporting the types in #178 but arrived at a simpler solution whereby we iterate &str and don't need to expose the underlying types. As a result, we removed the GeneralDnsNameRef type entirely as it was unused.

This issue is a reminder that we should consider restoring the GeneralDnsNameRef type and using it for its intended purpose: as part of the GeneralName enum.