Provide subscripts for JSON.Object to return the first key-value pair whose key matches a given key

[rgoldberg]

Provide subscripts for JSON.Object to return the first key-value pair whose key matches a given key.

Resolve #93

[tayloraswift]

i don’t think this is the right layer for this API. i believe there is a type JSON.ObjectDecoder which is designed for this purpose, and supports O(1), as opposed to O(n) access

[rgoldberg]

I parse JSON provided by a third-party that doesn't provide a spec.

I map various keys from the original JSON to new key names, and I sort the JSON based on the new key names.

I've never seen duplicate keys in the provided JSON, but, if duplicate keys exist, I want to map all of them & retain all duplicate keys.

I use JSONDecodable instead of JSONObjectDecodable because the latter uses JSON.ObjectDecoder, which seems to not support duplicate keys (please let me know if I'm wrong).

My internal processing must extract a few values from the input JSON.

The subscripts in this PR extract the first value listed in the JSON for a given key.

Does swift-json provide any better way to do what I need?

Here's an example of code that uses the subscripts from this PR:

extension SearchResult: JSONDecodable {
	fileprivate init(json: JSON.Node) throws {
		guard case let .object(object) = json else {
			throw MASError.jsonParsing(input: String(describing: json))
		}

		adamID = try object["trackId"]?.decode() ?? 0
		appStorePageURL = try object["trackViewUrl"]?.decode() ?? ""
		sellerURL = try object["sellerUrl"]?.decode()

		self.json = String(describing: json.mappingKeys)
	}
}

private extension JSON.Node {
	var mappingKeys: Self {
		// map keys & sort them
	}
}