cleanup depricated uint in rand/mod.rs and rand/os.rs

[Victory]

Cleanup mention of uint, use usize/us instead. This includes the example println!("{}", 2u * x);

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @pcwalton (or someone else) soon.

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[huonw]

I think it should be possible to just remove most of the suffixes, could you try that?

[Victory]

Yes i can remove those.

[alexcrichton]

@bors: r+ ac285d5 rollup

Thanks!