Create a PresentedDNSID type to represented DNS IDs in certificates

[briansmith]

When investigating an issue mentioned in #65 I thought "Why not change GeneralName::DNSName(untrusted::Input<'a>) to GeneralName::DNSName(DNSNameRef<'a>)?". I started down that path. However, I then was reminded by the code that GeneralName::DNSName is not a DNS name, but a "presented DNS ID." A presented DNS ID can either be a DNS name (DNSNameRef) or it could be a wildcard reference like "*.example.com", which is not a DNSNameRef. Thus we need a new type, something like:

enum PresentedDNSIDRef<'a> {
    DNSName(DNSNameRef<'a>),
    Wildcard(untrusted::Input<'a>),
}

Then GeneralName::DNSName(untrusted::Input) can be changed to GeneralName::DNSName(PresentedDNSID).

Note that this might cause us to recognize invalid DNS names earlier and we should be sure not to change our current behavior (whether it is skipping invalid DNS names or aborting the operation upon an invalid DNS name; I've forgotten).