Exercise 4.6: 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.