Add cartesian product algorithm.

Author: trekhleb

src/algorithms/math/cartesian-product/README.md

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+# Cartesian Product
+
+In set theory a Cartesian product is a mathematical operation that returns a set 
+(or product set or simply product) from multiple sets. That is, for sets A and B,
+the Cartesian product A × B is the set of all ordered pairs (a, b)
+where a ∈ A and b ∈ B. 
+
+![Cartesian Product of Two Sets](https://en.wikipedia.org/wiki/Cartesian_product#/media/File:Cartesian_Product_qtl1.svg)

src/algorithms/math/cartesian-product/cartesian-product.js

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+