In a few hours, the Boston Common will be awash in pink hats, protest signs, and a sea of demonstrators joining the Women’s March for America to speak out against hostile rhetoric and violence directed at immigrants, Muslims, and others during the presidential campaign and in the wake of the election. The Boston event is expected to draw tens of thousands of demonstrators and is one of more than 670 marches being held nationwide and globally, a day after President Trump formally took office.