Stack frames: Better edge destinations

[dwhatley19]

http://web.mit.edu/dwhatley/www/snapdown/web/production.html#%231%20-%3E%206%0Amain()%20%7Ba%20-%3E%20%231%7D%0Amain2()%20%7Ba%20-%3E%20%231%7D

Current result:

These two edges both go "up and to the right", so their destinations should be constructed appropriately (e.g. "down and to the left" of the destination object). Two arrows should also not go to the exact same place.