7: Add solution for Rust track exercise "Raindrops".

Created by  Aaw9nJhsNmfzFih9mKyNw9mV8CgERXJkRa1kK1Kx3LQH  on August 16, 2021
N3POGH26YQYAWWXIKGYBLDKGNM45LYCNGR74ATFYJOHQXP373WJQC
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