Ark std rand

[ValarDragon]

Description

Moves ark-ff::UniformRand to be a re-export of ark-std's version. Deprecates ark-ff::test_rng().

closes: #113

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• Wrote unit tests - n/a
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[ValarDragon]

That second commit shows that the deprecation is working as intended!

[Pratyush]

Hmm unfortunately due to how we handle warnings, this change will cause downstream crates to fail to compile (because we #[deny(deprecated)])

[ValarDragon]

Hrmm, its only our repos that deny deprecated, perhaps other downstream users keep it at warn?
(Also perhaps we should change from deny(deprecated) to warn(deprecated) anyway?)

(I'll update this repo for the change though)