* Add a procedure

(either <thunk>)

that calls thunk and returns its values wrapped into a Right except
for when <thunk> raises an exception.  In that case, wrap the error
object in a Left and return it.

The "inverse" of this procedure is `either-ref'. The name "either" is
probably not the best one.

Implementation:

(define (either thunk)
  (guard (c (else (left c))) (call-with-values thunk right)))

* Add maybe-let* that works like and-let* but can handle multiple
values and can accept #f values.

I would be willing to contribute an implementation in case Wolfgang
doesn't want to.