Add `JsonNode.asArray()` and `JsonNode.asObject()`

[victornoel]

In the same way that there is various methods to manipulate a JsonNode to one of its potential real type (String, boolean, etc), there should be asArray() and asObject() available.

Currently the only way to retrieve a JsonNode as an ObjectNode or an ArrayNode is with the two methods with(String) and withArray(String) (even though in 2.9 their return type is broken).

But for example when manipulating an ArrayNode, it is not possible to get its elements as ArrayNode or ObjectNode (or I didn't find the way to do it).

I would expect to be able, for example, to do:

for(JsonNode node: anArrayNode) {
  node.asObject().set("a", new TextNode("b"));
}

[cowtowncoder]

Makes sense. I think am including that as part of bigger package of improvements for JsonNode:

https://github.com/FasterXML/jackson-future-ideas/wiki/JSTEP-3

and the only (?) question there would probably be what to do with non-existing value (whether to throw exception or return MissingNode). This is bit different from with(), which automatically creates an entry. I think I'd go with MissingNode, and then have "required" variant that throws exception.

[cowtowncoder]

Oh. Actually, returning MissingNode makes no sense as result type would be expected to be ArrayNode / ObjectNode, respectively. So exception needed.