Ticket 392: Exact positive and non-negative integer predicates

There are two useful subsets of the exact numbers, both of which are commonly called natural numbers, depending on who's talking. Logicians, set theorists, and computer scientists include 0, other mathematicians mostly don't. This proposal adds the predicates exact-positive-integer? and exact-non-negative-integer?, analogous to exact-integer?. Because of the ambiguity, the name natural-number? is not proposed. Note that while exact-non-negative-integer? is a long name, call-with-current-continuation is even longer.