exercises: rebase, improve quasiquote and let*, avoid circular dependencies.

exercises: rebase, improve quasiquote and let*, avoid circular dependencies.

* Avoid including `lib/*.mal` to prevent unexpected circular
  dependencies (bitten by `foldr` using `nth`).
* Ask `quote` before `let*`, the latter requires the former.
  Answer previous questions without `let*`.
* Tell the reader that `let*` has various levels of difficulty.
* Drop implicit dependency on `or`, soon out of step files.
* Allow simple recursion in `let*` via a combinator.