Exercises 5.26, 5.27
Exercise 5.29
No tail-call optimization
Exercise 5.28
Revert "No tail-call optimization"

This reverts commit 9ce90a9b6644f8b39680c44fa7dfa58be59524db.
Duplicated ndpar-5.4.rkt -> ndpar-5.4-lazy.rkt
Lazy Explicit-Control Evaluator
Exercise 5.30.a
Exercise 5.30.b

A little bit cheating with Racket built-in error handling.