Added Factorial in Rust

[msj2]

accept a number on command line (works till 34)
print it's factorial.

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• I fixed Add Factorial in Rust #1830

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Notes

Accept a number on the command line and return the factorial of the number.

[msj2]

waiting.

[jrg94]

Looks close! We'll have to loop in some testing in the future to confirm, however.