Use nonnegative-integer? in favor of natural-number? #23

The natural-number? predicate seems to be
unavailable (though there is a remnant of it in the form of
natural-number/c, which is just a contract alias of
exact-nonnegative-integer?). In any case, it surely means
nonnegative integer.

(One might push the envelope here slightly and add
exactness, but this commit does not go that far.)

The natural-number? predicate seems to be unavailable (though there is a remnant of it in the form of [`natural-number/c`](https://docs.racket-lang.org/reference/data-structure-contracts.html?q=nonnegative-integer%3F#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._natural-number%2Fc%29%29), which is just a contract alias of [`exact-nonnegative-integer?`](https://docs.racket-lang.org/reference/number-types.html?q=exact-nonnegative-integer%3F#%28def._%28%28quote._~23~25kernel%29._exact-nonnegative-integer~3f%29%29)). In any case, it surely means nonnegative integer. (One might push the envelope here slightly and add exactness, but this commit does not go that far.)

They should all be exact. And now they are. Thanks!

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Use nonnegative-integer? in favor of natural-number? #23

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