6: Add solution to Rust track exercise "Gigasecond".

Created by  Aaw9nJhsNmfzFih9mKyNw9mV8CgERXJkRa1kK1Kx3LQH  on October 10, 2021
KG7NGGS4TDAGDVHKJ4A5CHLRXQN2HBVZSDJNA7WIORMFRBX7YIRAC
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