Keep a trailing AS set out of the origin and the peerings (closes #75)

A path that ends in an AS set of several members, such as
[64496, 64497, [64498, 64499]], now has no origin: its route is stored
with origin 0 (none), and no member of the set is in a peering. A set of
one member counts as that AS. An announcement with an empty path is
still not stored.

ASPath keeps such a set apart from the sequence and gives the origin.
The prefix and AS handlers take the origin from it; the peering handler
uses the sequence only. The stored AS path still lists the set's members
at the end.

The prefix and prefix length pages show origin 0 as no known origin, the
IP lookup asks no WHOIS lookup for it, and the README says what it means.

Model: opus-5-5

internal/templates/prefix_length.html

@@ -172,9 +172,13 @@
                 <td><a href="{{ .Prefix | prefixURL }}" class="prefix-link">{{ .Prefix }}</a></td>
                 <td class="age">{{ .Age }}</td>
                 <td>
+                    {{ if .OriginASN }}
                     <a href="/as/{{ .OriginASN }}" class="as-link">
                         AS{{ .OriginASN }}{{ if .OriginASDescription }} ({{ .OriginASDescription }}){{ end }}
                     </a>
+                    {{ else }}
+                    No known origin
+                    {{ end }}
                 </td>
             </tr>
             {{ end }}