[Geojson] Being more specific about immutable semantics

[Sean Gillies <[e-mail]>]

We've got consensus that extensions must not change the semantics of
GeoJSON types.

https://github.com/geojson/draft-geojson/blob/master/middle.mkd#semantics-of-geojson-properties-and-types-are-not-extensible

I'd like this to be more specific to steer implementers away from producing
monsters like this:

{
  "features":[
    {
     "features": [],
      "geometry": {
      "coordinates": [0, 0],
        "type": "Point"
    },
    "properties": {
     "name": "Null Island"
    },
    "type": "Feature"
    }
    ],
  "geometry": {
    "coordinates": [123.0, 345.0],
    "type": "Point"
  },
  "properties": {
    "name": "Dinagat Islands"
  },
  "type": "FeatureCollection"
}

It's a FeatureCollection with semantics of Feature and Geometry containing
a Feature with semantics of FeatureCollection.

Concretely: I propose to add to the "Extending GeoJSON" section a
definition of the GeoJSON properties, enumerating "id", "type", "features",
&c. And add to the "Semantics of GeoJSON properties and types are not
extensible" section something like the following:

- A Feature or Geometry object MUST NOT contain a "features" property.
- A FeatureCollection or Geometry object MUST NOT contain a "geometry" or
"properties" object.
- A FeatureCollection or Feature object MUST NOT contain a "coordinates"
property.

I'll prepare a PR for review unless there are objections.

-- 
Sean Gillies