STUCK IN THE PLACE WHERE YOU ARE BORN                 The notion that people should be able to choose their own communities - instead of  being  stuck wherever they happen to be born - is America's most profound contribution to  the world. Many of  our country's most glaring injustices result from the ways in which this  freedom has been denied to  those who need it most. America is a nation of migrants. At the  end of 19th century roughly 1/3 of all  Americans changed addresses each year. On average  migrants have always grown more prosperous  than those who have stayed in one place.  Mobility has been the great engine of American prosperity.  But for the last 50 years, the  engine of American opportunity has been grinding to a ha...