Fragile pattern warning 4

[Alexander Ryzhikov]

Using decco outputs this warning for every usage

It will remain exhaustive when constructors are added to type Js_json.tagged_t.
  
  Warning number 4
  (No file name)
  
  this pattern-matching is fragile.

[Alexander Ryzhikov]

@ryb73 #42 fixes it, could you please check it? :)

PS whole code base is not formatted with refmt, I can fix this too if needed