Rename enum items in ChallengeGenerator

[autquis]

Description

The names of the enum items in ChallengeGenerator are a bit confusing. This PR is a suggestion for renaming them. I think Independent is a more descriptive name than Multivariate. Ditto for Correlated.

This PR is on top of #137

Pointed out by @mmagician

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[Pratyush]

I think we should get rid of this enum entirely and replace it with squeezing from the sponge. (Or maybe @mmaker's new crate that handles Fiat-Shamir E2E).

Given this, I am somewhat reluctant to merge this PR, and would instead recommend directing efforts towards replacing the ChallengeGenerator usage with sponge.squeeze. What do you all think?

[mmagician]

Oh yes, that would make our lives much easier, especially for the refactors coming in later .