Multi-step diagrams with a static stack, reassigned variables

[dwhatley19]

Currently completely broken:
http://web.mit.edu/dwhatley/www/snapdown/web/production.html#main()%20%7B%0Aa%20-%3E%202%0A%7D%0A---%0Aa%20-%3E%205

A good first step to incorporating stack frames into multi-step diagrams would be to require the stack to be completely fixed, and only allow heap-type reassignments.