Dict: Add ToString() and ToJSON()

[kenorb]

Expand Dict class to add ToJSON() method (similar to ToString()). The method should return all elements in the plain JSON format.

E.g.

  // ToJSON()
  Dict<string, int> dict1;
  dict1.Set("a", 1);
  dict1.Set("bb", 2);
  Print(dict1.ToJSON()); // {"a": 1, "bb": 2}

  // ToString()
  Dict<string, int> dict2;
  dict2.Set("a", 1);
  dict2.Set("bb", 2);
  Print(dict2.ToString(";")); // a=1;bb=2
  Print(dict2.ToString("\n")); // a=1\nbb=2

We can either implement as the self-contained method, or create a JSON class as a wrapper (as ToJSON() could be also re-used in other classes).

Not sure if we can have multiple hierarchy such as Dict<string, Dict> dict; (depending on the complexity), so for now we can only support the flat list.

Tasks:

• [x] Adds Dict's ToString() method to return all elements in the plain format (pass argument to specify delimiter string, e.g. ";" or "\n" for each item).
• [x] Adds Dict's ToJSON() method to return all elements in JSON format.
• [x] Optionally implement multiple hierarchy support.
• [x] Optionally implement a new JSON class if required.
• [x] Expand DictTest.mq5 to cover the test for the new method.

[kenorb]

Fixed in #102