NetTopologySuite.IO.GeoJSON 2.0.0

1. Now targets .NET Standard 2.0 only
2. CRS model objects are gone (only pre-IETF versions of GeoJSON used them exactly that way).
3. "id" is now fetched via an extension method that we add to IAttributesTable.
4. Added helper methods to GeoJsonReader and GeoJsonWriter to support streaming.
5. A few improvements to hopefully reduce the overhead of reading / writing geometries, especially those that use coordinate sequences other than CoordinateArraySequence, especially when streaming.
   • ed.: I think there's a lot of potential to do better here across-the-board for all kinds of geometries, but it was a really long weekend...