Exercise 5.39: Write a procedure
lexical-address-lookup that implements the new lookup operation.  It
should take two arguments—a lexical address and a run-time environment—and
return the value of the variable stored at the specified lexical address.
Lexical-address-lookup should signal an error if the value of the
variable is the symbol *unassigned*.331 Also write a procedure
lexical-address-set! that implements the operation that changes the
value of the variable at a specified lexical address.