Exercise 5.41: Write a procedure
find-variable that takes as arguments a variable and a compile-time
environment and returns the lexical address of the variable with respect to
that environment.  For example, in the program fragment that is shown above,
the compile-time environment during the compilation of expression ⟨e1⟩ is
((y z) (a b c d e) (x y)).  Find-variable should produce


(find-variable 
 'c '((y z) (a b c d e) (x y)))
(1 2)

(find-variable 
 'x '((y z) (a b c d e) (x y)))
(2 0)

(find-variable 
 'w '((y z) (a b c d e) (x y)))
not-found