A new study has found that the disparity between the haves and the have-nots is mounting in Boston’s northern suburbs as a growing number of local families struggle to overcome economic and social inequities. The report, unveiled at a Harvard Law School forum on Tuesday, shows the region’s wealth has become increasingly concentrated over the past three decades. What once was a gap between the haves and have-nots is now a chasm, leaving more families than ever unable to make ends meet.