TODO

      - Implement i/o ports as specified in r7rs small, and adapt the socket functions
        to use the port interface

      - implement exception handlers as specified in r7rs
        - dynamic environment = not lexically scoped?
          possibly store pointer to the current exception handler in stack frames

      - rewrite parser to do iterative parsing, so (read) can be implemented correctly
        - maybe complete that toy LL(1) parser, and use it to generate parsing rules

      - Reimplement tri-color marking algorithm in GC. Once all objects are marked,
        append the free list to a cache list which gets used for object allocation.
        This should be much faster, since only live objects would ever have to be
        traversed during GC cycles, and memory would be reused rather than having
        to go to the malloc()/free() repeatedly.
        Could also open the possibility of doing compacting.

      - Clean up source a bit, add useful comments

      - Add more bytevector functions as specified in r7rs

      - Begin working on a C FFI
        - idea for FFI: have a 'messaging' or 'event' interface,
          which can have multiple scheme functions registered to some key.

DONE  - Make garbage collection more efficient
DONE  - implement tail call elimination
DONE  - implement continuations