Add NAT_PLUS as a fully reversible relation

[robsimmons]

INT_PLUS can only be run backwards with a single unknown, because plus X Y is 12 has an infinite number of answers (X = 12 and Y = 0, X = 112 and Y = -100, and so on). But NAT_PLUS can be run backwards the same way STRING_CONCAT now can, because plus X Y is 12 only has 13 solutions.