Between the rise of e-commerce and the U.S. population's demographic shift to urban centers, the era of thriving big box retail is largely over. Along with evidence that U.S. retail is overstored, many merchants are left scratching their heads about what to do with excess space. Staples has one answer. The office supplies retailer recently announced that it would partner with office-space sharing startup Workbar to set up co-working spaces in three suburban Boston area stores.