Exercise 4.13: Scheme allows us to create new
bindings for variables by means of define, but provides no way to get
rid of bindings.  Implement for the evaluator a special form
make-unbound! that removes the binding of a given symbol from the
environment in which the make-unbound! expression is evaluated.  This
problem is not completely specified.  For example, should we remove only the
binding in the first frame of the environment?  Complete the specification and
justify any choices you make.