Add changes to allocator. Nowhere near finished.
Add changes to allocator. Nowhere near finished.

- For now, we'll use a simple mark-and-sweep GC usable with a generic malloc
  implementation. Simplifies issues with object layout & mutability.

- TODO: generational GC (especially since we should wind up with relatively
  few old objects pointing to new objects).

- TODO: pull typechecking out of object allocation (what was I thinking?)

bassettmb/scheme