Exercise 3.24: In the table implementations
above, the keys are tested for equality using equal? (called by
assoc).  This is not always the appropriate test.  For instance, we
might have a table with numeric keys in which we don’t need an exact match to
the number we’re looking up, but only a number within some tolerance of it.
Design a table constructor make-table that takes as an argument a
same-key? procedure that will be used to test “equality” of keys.
Make-table should return a dispatch procedure that can be used to
access appropriate lookup and insert! procedures for a local
table.