Finished the refactoring of separate pair types to vectors. Removed
LDCAR/SET_CAR/LDCDR/SET_CDR instructions. Added a STORE instruction for
storing global variables, using SET for aggregates (string/vector).
GET_BOUND_LOCATION now embeds the symbol directly into the opcode which
seemed like a good idea at the time I made the change. The specialization
of arguments was removed and now slots are used for both args and local
variables. Removed the get_arg_index function. Re-did the code behind
first/rest. Refactored some of the code that compiled symbol lookups.