7: Add Rust track exercise "Clock".

Created by  Aaw9nJhsNmfzFih9mKyNw9mV8CgERXJkRa1kK1Kx3LQH  on October 11, 2021
FMWRN4JLPDYGDFTG3Q6CNMHBABUKMSWJUORVXXTAPNSX5FADG3ZAC
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