Consider runtime-specified value "template" value for missing fields (or patching)

[kperryua]

Based on forum discussions here, consider capabilities like this:

struct BlogPost {
    var required: Int
    var items: [String] = []
}

// { "required" : 1 }
decode(template: BlogPost(required: 0), from: data)
// -> BlogPost(required: 1, items: [])

// { "required" : 1 }
decode(template: BlogPost(required: 0, items: ["world"]), from: data)
// -> BlogPost(required: 1, items: ["world"])