12: Improve solution for Rust track exercise "Nth Prime".

Created by  Aaw9nJhsNmfzFih9mKyNw9mV8CgERXJkRa1kK1Kx3LQH  on August 17, 2021
UTP7D53QIV7KUBI3DX36GSDXMSAVIMDSPOSYN7SJEREOHBHJIEKAC
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