Add Unit Testing:
 - Add unit test for arithmetic operations
Add unit tests for list functions.
Add quote tests.
Fix quote display bug.
Refactor evaluation and testing.
Fix bug when 'quote' does not have an argument:
 - Don't try to insert nil value into array.
Add test cases for ' (shorthand quote).
Fix bug with expand error reporting.
Add display checks to arithmetic unit tests.
Add display checks to list unit tests.
Add inequality unit tests.
Add boolean unit tests.
Add length unit test to list tests.
Validate number of arguments for length function.
Validate number of arguments for append function.
Add unit tests for append function.
Add comparator unit tests.
Use NSMutableString instead of NSString for string literals:

NSString is a class cluster that delivers __NSCFConstantString
for constant strings.  This means that separate instances of
a constant string get represented by the same object, which
causes, e.g.
  (eq? "x" "x")
to evaluate to #t instead of #f.
Add one more eq? unit test.
Add if unit tests.