Statically allocate empty vector instead of using null pointer with type tag

[pdarragh]

Per discussion in #38, we should move toward statically allocating the empty vector and having a single unique value for it rather than using the type tag in conjunction with the null pointer.

[dvanhorn]

This is an easy change to make to the compiler, but requires a significant reworking of the notes since static allocation needs to come before (or at the same time as) dynamic allocation. I think the right approach is to cover quoted lists first, then add cons, etc.

[pdarragh]

Right; I think this falls in-line with some other changes we'd proposed for next year about rearranging the course material, especially with regard to memory stuff. Maybe we can get that in place for next fall?