Implementation of planout interpreter in Golang

```
       "value": {
        "values": [
          {
            "op": "get",
            "var": "numbers"
          }
        ],
        "op": "length"
      }
```

I think I should just be able to do this:
```
values := evaluate(m["values"], s.params).([]interface{})
return len(values[0])
```