4: Add solution to Rust track exercise "Reverse String".

Created by  Aaw9nJhsNmfzFih9mKyNw9mV8CgERXJkRa1kK1Kx3LQH  on October 10, 2021
TZGS3ALL3ZK2TDIN3GT2OGW4QZLXV55BOHOM2NQ25AXKS5YP4YGAC
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