6: Add Rust track exercise "Raindrops".

Created by  Aaw9nJhsNmfzFih9mKyNw9mV8CgERXJkRa1kK1Kx3LQH  on August 16, 2021
MSFESBT6OSZ23YMSEOMONHXQ3RKPM3FGLC3D4AEULXBXVHKQ4GMAC
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