Factored out STDOUT and STDERR Streams, adding to Fields in Output JSON

Split `stdout` and `stderr` streams, now output has this format (for example):

```JSON
{
    "output": {
        "fooPrinter": {
            "is pretending to be the pure value \"Bar\"": {
                "success": true
            }
        },
        "hammings": {
            "starts with [1,2,3,4,5]": {
                "success": true
            }
        }
    },
    "stderr": "",
    "stdout": "Foo",
    "success": true,
    "time": 0.002149
}
```