Add function for unbiased random numbers

[kurnevsky]

Later it will be used to choose random nodes for onion paths

[coveralls]

Pull Request Test Coverage Report for Build 1984

• 13 of 14 (92.86%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.004%) to 96.205%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/toxcore/crypto_core.rs	11	12	91.67%

Totals
Change from base Build 1981:	-0.004%
Covered Lines:	13537
Relevant Lines:	14071

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💛 - Coveralls

[kpp]

https://rust-random.github.io/rand/src/rand/distributions/uniform.rs.html#403

Would you please check it?

[kurnevsky]

@kpp they do the same but with multiplication instead of modulo. Honestly, I don't know why. Perhaps it's more performant even though much more complex. In terms of correctness my version should be fine. Alternatively we can use rand crate instead of libsodium but I'd left it for the future.

[kurnevsky]

@kpp right, it's because of performance: rust-random/rand#570 (comment)

[kpp]

rebase