Major refactorings

  - New data type Null with single value Empty
  - Don't use nils in cons lists anymore (should die if nil is  discovered)
  - Parse dotted pair syntax '(a . b)
  - reimplement cons, car, cdr with single return value and
    return error as a Data. Experiemental to see if it makes sense to
    have an Error type or stick with multi-value returns from eval.
  - Ignore ;; comments
  - implement let