Dumper: dump WeakMap entries

[cpcallen]

If we're going to make more use of WeakMaps in the core—especially, to hold persistent information rather than just as caches—then Dumper needs to dump their state.

Expected work:

• Preparatory refactoring:
  • Dump-by-object, with catch-up lists?
  • Survey: diff?
  • More TBD?
• Selector: need SpecialPart(s) for WeakMap entries.
• Temporary names.
• WeakMapDumper.
  • Dump WeakMap entries.