Added support for Pairs via the 'Pair.new()' functionality and 'first':'second' syntax.

Example:
  var array_of_pairs = [
      red:   9,
      blue:  4,
      green: 0,
  ];

  say array_of_pairs[0].first;    # prints the first element: 'red'
  say array_of_pairs[0].second;   # prints the red's value: 9