Split out next and child handling from top-level `_DUMP`

[fredemmott]

• add _DUMP_WITHOUT_NEXT for use in callsites where next is already being dumped
• _DUMP should be implemented by calling _DUMP_WITHOUT_NEXT then _DUMP_NEXT, if there is a next field
• _DUMP_WITHOUT_NEXT should delegate to known child struct DUMP_WITHOUT_NEXT depending on type