v4.0.0-pre.5

curve25519-dalek is a library providing group operations on the Edwards and
Montgomery forms of Curve25519, and on the prime-order Ristretto group.

Breaking changes in 4.0.0

• Update the MSRV from 1.41 to 1.56.1
• Update backend selection to be more automatic
• Remove std feature flag
• Remove nightly feature flag
• Make digest an optional feature
• Make rand_core an optional feature
• Replace methods Scalar::{zero, one} with constants Scalar::{ZERO, ONE}
• Scalar::from_canonical_bytes now returns CtOption
• Scalar::is_canonical now returns Choice
• Deprecate EdwardsPoint::hash_from_bytes and rename it
  EdwardsPoint::nonspec_map_to_curve
• Require including a new trait, use curve25519_dalek::traits::BasepointTable
  whenever using EdwardsBasepointTable or RistrettoBasepointTable

This release also does a lot of dependency updates and relaxations to unblock upstream build issues.