Still untested: since we only use 15 bits for the sequence number
delta, it's _possible_ (but practically impossible) to encounter
events that don't fit the format.  For these, insert dummy non-taken
branches (they will have a destination of 0 and can be ignored).

{
  "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

Example header:

  {
          "title": "TinyEMU Branch Trace",
          "format": [
                  {
                          "field": "was_taken",
                          "bits": 1
                  },
                  {
                          "field": "insn_delta",
                          "bits": 15
                  },
                  {
                          "field": "branch_addr",
                          "bits": 48
                  }
          ],
          "date": "20210418"
  }
  Args: "./temu" "boot-linux.cfg"
  Launched: 1618780223
  Ended: 1618780226
  Duration: 3
  Branches: 75130370
  Instructions: 478005076
  Avg insn / branch:   6.36
  Avg MIPS: 159.34
  Avg taken ratio:   0.76