Disable Rand Dependency When Feature Unused

[ash-burnt]

Description

The rand and getrandom libraries are a particular nuisance when trying to compile Rust to certain architectures, like wasm.

This PR sets the rand dependency to optional, meaning it will not enter the dependency tree when features don't call for it, and should allow wasm compilation to complete

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[Pratyush]

Hi, can you switch it so that maybe we keep rand, but basically avoid getrandom, if that's the offending dependency?

I think you can do this by changing rand to have no-default-features set, and then selectively enabling the std_rng feature?

[ash-burnt]

This keeps rand, just sets it as an optional dependency if none of the features need it. If you don't have a feature using it, it won't get pulled in and won't cause issues

Most dependencies should be optional, this is an irk across most of the arkworks tools. For something like verification only, we still end up pulling in most of the unused dependencies