[FR] JSON AoS<->SoA redesign

Currently, we emit each repetition separately into JSON (and all at once apparently).
This is quite bloaty.

If we ever want to support iteration-as-repetition thing (and i think we do),
and perhaps if we want to AoS<->SoA our internal structs (and i think we do, especially if we do iteration-as-repetition),
i think we need to change the JSON schema.
The numberic fields (times, counters) need to become arrays of integers.

The `tools/` will support both schemes, but external tooling will need similar updates.
I don't think we'd want to support two different json formats?

@dmah42 thoughts?