Writable JSON API changes (dotnet/corefx#40673)
* implicit operators moved to JsonNode
* JsonNull introduced
* JsonObject rewritten to use JsonObjectProperty
* JsonElement.ObjectEnumerator changed to use _current instead of _enumerator
* iterative DeepCopy implemented
* code reorganized
* parse implemented
* JsonDocumentOptions added to JsonNode.Parse
* WriteTo and ToJsonString added
* specification adjusted
* string comparison added
* JsosReadonlyDomWriteTests reused
* code coverage improved
* minor other fixes
* review comments included
Commit migrated from https://github.com/dotnet/corefx/commit/
37b37395481b635c15eaa3454e52b72ab91de4ef