Strange behavior with callbacks

[trezm]

Take this runkit as an example. Basically if the argument is passed to a callback and is not undefined, you get the error

[
  {
    "name": "TypeMismatchError",
    "message": "Type mismatch in declaration on line 9",
    "extras": {
      "actualType": "Number",
      "expectedType": "String",
      "file": [removed for brevity],
      "start": {
        "line": 9,
        "column": 10
      },
      "end": {
        "line": 9,
        "column": 48
      }
    }
  }
]

[trezm]

Fixed by #44