Summarizing conversation with Parker and Ian here, things to consider include

• A single buffer for field names
• A single buffer of (offset, tag, value) tuples where 'offset' references the field name buffer.

Ideally this object would be traversable without calling strlen on field names.

While we're in the area, we could delete the reserve() call in push_back_raw().

We should audit the callsites and usage patterns before coming to a decision here.