Validator needs to prohibit explicit exponential functions for piecewise-constant style properties

[jfirebaugh]

This should be an error:

    {
      "id": "invalid explicit exponential function",
      "type": "fill",
      "source": "source",
      "source-layer": "layer",
      "paint": {
        "fill-antialias": {
          "property": "mapbox",
          "type": "exponential",
          "stops": [
            [
              0,
              true
            ],
            [
              1,
              false
            ]
          ]
        }
      }
    }