3: Add Rust track exercise "Reverse String".

Created by  9Zb2bmkejrNknawUtr3MKvVstZkVDR8x8ritfgZXKrky  on December 23, 2021
FNQ7GM3XFJDHK4QMBSZCYDK5GZ5C7P2MNYXKAKID2NAFGVHECUMQC
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