“Dartmouth was, however, formed as a corporation, and the central question in the college’s case had a direct bearing on all corporations, business or otherwise: Are corporations public or private? Are they public entities subject to broad governmental control? Or are they private entities over which the state has only limited regulatory authority? Today, we take for granted that corporations are private entities. In the early 1800s, however, the question of whether corporations were public or private remained unsettled. Webster’s Dartmouth College case would largely settle it.”