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

Created by  9Zb2bmkejrNknawUtr3MKvVstZkVDR8x8ritfgZXKrky  on December 23, 2021
SYZUBDQSNIMLWM4BBHJNB6MOFMXTZBR3AX6SRW3S6G4B6UGLVQCQC
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