Have `GenericRouterStatus` use less RAM.

We need to allocate a bunch of RouterStatus objects for our directory parsing code, so it would be great to have them be smaller. Some possibilities include:

• using smolstr or smartstr for nicknames and/or versions.
• storing most versions in parsed form?
• Using shortvec or similar to store addrs.
• Removing unused or_port field.

These show up in memory profiles for directory info under #87.