We were using the public suffix of the URL's host as its registrable
domain. But the registrable domain is actually the public suffix plus
one additional label.
Specifically, after implementing some recent spec changes to navigables,
we end up calling `get_public_suffix("localhost")` here, which returns
OptionalNone. This would previously crash.
Our get_public_suffix() seems a little incorrect. From the spec:
> If no rules match, the prevailing rule is "*".
> https://github.com/publicsuffix/list/wiki/Format#algorithm
However, ours returns an empty Optional in that case. To avoid breaking
other users of it, this patch modifies Host's uses of it, rather than
the function itself.
This lets us move a few Host-related functions (like serialization and
checks for what the Host is) into Host instead of having them dotted
around the codebase.
For now, the interface is still very Variant-like, to avoid having to
change quite so much in one go.
