frost-core v2.0.0-rc.0

• Changed the deserialize() function of Elements and structs containing
  Elements to return an error if the element is the identity. This is a
  requirement in the FROST specification that wasn't being followed. We are not
  aware of any possible security issues that could be caused by this; in the
  unlikely case that the identity was being serialized, this would be caught by
  deserialization methods. However, we consider this change the right thing to
  do as a defense-in-depth mechanism. This entails the following changes:
  • Group::serialize() now returns an error. When implementing it, you must
    return an error if it attempts to serialize the identity.
  • VerifyingShare::serialize(), CoefficientCommitment::serialize(),
    VerifiableSecretSharingCommitment::serialize(),
    NonceCommitment::serialize(), Signature::serialize(),
    VerifyingKey::serialize() can now all return an error.
• Changed the serialize() and deserialize() methods of all Scalar- and
  Element-wrapping structs; instead of taking or returning a
  Field::Serialization or Element::Serialization trait (which are usually
  defined by ciphersuites as arrays of specific sizes), they simply respectively
  take &[u8] and return Vec<u8>, exactly as the other structs, which should
  greatly simplify non-serde serialization code. You can port existing code with
  e.g. x.serialize().as_ref() -> x.serialize() and
  X::deserialize(bytes.try_into().unwrap()) -> X::deserialize(&bytes).
• Removed the ops::{Mul, MulAssign, Sub} implementation for Identifier.
  These were being used internally, but library users shouldn't need to use them.
  If you have low-level code that relied on it, use Identifier::{new, to_scalar} to handle the underlying scalar.
• Removed batch::Item::into() which created a batch Item from a triple of
  VerifyingKey, Signature and message. Use the new batch::Item::new() instead
  (which can return an error).
• Add no-std support to all crates except frost-ed448. To use, do not enable the
  std feature that is enabled by default (i.e. use default-features = false); Note that it always links to an external alloc crate (i.e. there is
  no alloc feature). When disabling std, the only impact in the API is that
  Error will no longer implement the std::error::Error trait. This is a
  breaking change if you are disabling default features but rely on Error
  implementing std::error::Error. In that case, simply enable the std
  feature.
• Fixed no-default-features, previously it wouldn't compile.
• Fixed some feature handling that would include unneeded dependencies in some
  cases.