- Add untested named-let;

- Add `if`;

- Add `lambda` with partial support;

- Fix step-check, to recurse into expressions;

- Add step-body->letrec* that will translate a body into a letrec*;

- Add step-letrec*->nested-let+set! that will translate letrec*
  into nested let, and set!;

- Fix step-let-as-lambda to recurse into body, and arguments;

- Improve step-continuation-passing-style: add support for void, set!,
  and if;

- Demonstrate the use of call/cc with make-coroutine-generator;
step-let-as-lambda, step-continuation-passing-style with call/cc...

... with some debugging helpers (see trace-define).
step-check: initial version.