Group trait overhaul part 2

[daxpedda]

Follow-up of #52.

• Remove our implementation of expand_message_xmd and use ExpandMsgXmd of elliptic-curve instead.
• Update MSRV to 1.57 and Rust edition to 2021
• Re-introduce CipherSuite and move SUITE_ID to there. This allows consumers to implement their own group and hash combination and assign a custom suite id.
• Introduce types-defs that represent the serialization length of voprf types.
• Blanket-implement Group for RustCrypto Curves.
• Remove custom Group implementation for NistP256.
• Rename ristretto255_* crate features to ristretto255-*.

We should probably properly document CipherSuite and the ristretto255-ciphersuite crate feature. I hope you like these changes @kevinlewi, I couldn't find any other way to properly do this.

This is still waiting for RustCrypto/traits#906 and RustCrypto/elliptic-curves#515.

[daxpedda]

CI failures have nothing to do with the PR, just connection issues while updating Rust.

[kevinlewi]

Awesome.