Implement a syntactic transformation

Let expressions are derived expressions, because

(let ((var1 exp1) ... (varn expn))
  body)

is equivalent to

((lambda (var1 ... varn)
   body)
 exp1
 ...
 expn)

Implement a syntactic transformation let->combination that reduces evaluating let expressions to evaluating combinations of the type shown above, and add the appropriate clause to eval to handle let expressions.