Bump rand to 0.8 and remove rand_xorshift

[weikengchen]

This is currently a draft PR, since there are still a number of libraries in arkworks-rs that needs to be adjusted to ark_std::rand, including the public repos crypto-primitives, curves, pcd and some private repos.

Since we have test_rng, it is planned to retire rand_xorshift.

[Pratyush]

btw @weikengchen , any updates on this?

[weikengchen]

Let me do a final pass on existing repos under arkworks.

[weikengchen]

All repos in arkworks have been modified, and I think it is ready to bump the version.

Note that after this PR, some post-merging actions are needed---I doubt rand_chacha 0.2 would work for rand 0.8, and we may need to bump it to rand_chacha 0.3 after this PR.

[weikengchen]

Since I don't have write access to this repo and it seems that this PR needs to sync with the master updates...I would leave this one to @Pratyush for a final pass.

[weikengchen]

(Bump)

[weikengchen]

Let me close this one and recreate it (the conflicts cannot be resolved in the web editor).