Json: Error: type mismatch: got '...' but expected '(string, string)'

[Vindaar]

The issue you're seeing is simply that you reused the `%*` identifier for your 
custom `JsonNode` converter. It confuses the compiler. `%*` is an `untyped` 
macro in the JSON module, which applies `%` to every field essentially.

So for your own JSON converter procs for a type, you'll want to use `%` as 
well. A working example:
    
    
    import std/json
    
    type
      Planet* = ref object
        name*: string
        pos*: tuple[x: float, y: float]
    
    proc `%`(planet: Planet) : JsonNode =
      result = %* {
        "name": planet.name,
        "pos": [ planet.pos.x, planet.pos.y ]
        }
    
    let p = Planet(name: "Earth", pos: (x: 149.6e9, y: 0.0))
    echo pretty(% p)
    
    
    Run