It’s not too hard to get to TD Garden from Agganis Arena, on the campus of Boston University. A little ways down Commonwealth Avenue, then Storrow Drive takes you almost the rest of the way there. From being in the shade of a massive 26-story dormitory to the neighbor of a major highway and modern bridge, the differences are stark. The differences that newly signed prospects Charlie McAvoy and Jakob Forsbacka-Karlsson could make are even larger. McAvoy, a big bodied offensive-defenseman, and JFK (as he’s known to Terriers fans), a strong two-way center, are the latest prospects to sign with the Bruins.