3: Add Rust track exercise "Reverse String".

Created by  Aaw9nJhsNmfzFih9mKyNw9mV8CgERXJkRa1kK1Kx3LQH  on October 10, 2021
W2V5PYAQ6CH5ALKJCNZOT4T5DGBL3BAPVHXSRR7IJASPYTWWFHIAC
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