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.
Fix bug when EOF is read mid symbol.
Add set! unit tests.
Fix syntax display for set!.
Fix set! bug when trying to set an undefined symbol.