Recursive data structures are broken

[pdarragh]

Related to #10.

The ziggy bits->value functions recurse without pause. Consider the case for vectors in Hoax:

https://github.com/cmsc430/www/blob/d6fbb58e6062b6723a159a168b48cff128d542cc/ziggy/src/types.rkt#L44-L50

I think bits->value could be reworked to include a map from addresses to reconstituted values so we can reconstruct cyclical heap-allocated data structures.

Depending on how complex the final implementation is, it might give us an opportunity to talk about cyclical data structures explicitly in the course.