Added cons-list iterator.
Fixed unit tests accessing fields directly.
car and cdr are private, switched add to use for each over list.
Subtraction works with list iterators.
Multiplication uses iterators.
Division uses iterators.
Equality operator works with iterators.
If procedure uses cons_cell accessors.
Quote procedure uses cons_cell accessors.
Set procedure uses cons_cell accessors.
Define uses iterators and accessors for cons_cells.
Lambda uses iterators.
Begin uses cons_cell iterators.
Let uses iterators and accessors for cons_cell.
Display uses cons_cell iterators.
Greater than operator uses iterators and accessors for cons_cell.
Less than operator uses iterators and accessors for cons_cell's.
Re-enabled intergration tests.