31: Improve solution for Rust track exercise "Raindrops".

Created by  Aaw9nJhsNmfzFih9mKyNw9mV8CgERXJkRa1kK1Kx3LQH  on September 2, 2021
XDEY7SNLZMAC3KIKUHDGZGGXHHT7BYF3GSVAREXESO5D4S6FZBQQC
Change contents