function KeySequence(parent) {
    this.parent = parent;
  }
Returns a {@link Sequence} whose elements are the keys of this object-like
sequence.

@public
@returns {Sequence} The sequence based on this sequence's keys.

@examples
var obj = { hello: "hola", goodbye: "hasta luego" };

Lazy(obj).keys() // sequence: ["hello", "goodbye"]
Lazy(obj).keys().map(function(v, i) { return [v, i]; }) // sequence: [["hello", 0], ["goodbye", 1]]