Kraken, one of the world’s largest cryptocurrency exchanges, on Wednesday announced the layoff of 15 percent of its workers, a corporate restructuring and the appointment of a new co-chief executive. The cuts amounted to about 400 of the company’s roughly 2,600 employees. They included two members of the leadership team: the chief operating officer, Gilles BianRosa, and the chief technology officer, Vishnu Patankar.