function ConsecutiveSequence(parent, count) {
    this.parent = parent;
    this.count = count;
  }
Groups this sequence into consecutive (overlapping) segments of a specified
length. If the underlying sequence has fewer elements than the specified
length, then this sequence will be empty.

@public
@param {number} length The length of each consecutive segment.
@returns {Sequence} The resulting sequence of consecutive segments.

@examples
function sum(vals) { return Lazy(vals).sum(); }
var pairs = Lazy([1, 2, 3, 4]).consecutive(2);

// Make sure consecutive sequences are reusable.
pairs.map(sum) // => sequence: [3, 5, 7]
pairs.map(sum) // => sequence: [3, 5, 7]

Lazy([]).consecutive(2)        // => sequence: []
Lazy([1]).consecutive(2)       // => sequence: []
Lazy([1, 2]).consecutive(2)    // => sequence: [[1, 2]]
Lazy([1, 2, 3]).consecutive(2) // => sequence: [[1, 2], [2, 3]]
Lazy([1, 2, 3]).consecutive(0) // => sequence: [[]]
Lazy([1, 2, 3]).consecutive(1) // => sequence: [[1], [2], [3]]