Make branch trace header all-JSON

{
  "title": "Branch Trace",
  "format": [
    {
      "field": "was_taken",
      "bits": 1
    },
    {
      "field": "insn_delta",
      "bits": 15
    },
    {
      "field": "branch_addr",
      "bits": 48
    }
  ],
  "date": 20210418,
  "args": [
    "./temu","bool-linux.cfg"],
  "launch": 1618801656,
  "finished": 1618801657,
  "duration": 1,
  "branches": 15148555,
  "instructions": 117501736,
  "avg insn / branch":  7.757,
  "avg MIPS": 117.502,
  "avg taken ratio":  0.485
}

Slightly overkill, but it might be useful in future