Exercise 3.3: Modify the make-account
procedure so that it creates password-protected accounts.  That is,
make-account should take a symbol as an additional argument, as in


(define acc 
  (make-account 100 'secret-password))

The resulting account object should process a request only if it is accompanied
by the password with which the account was created, and should otherwise return
a complaint:


((acc 'secret-password 'withdraw) 40)
60

((acc 'some-other-password 'deposit) 50)
"Incorrect password"