frost-core v0.3.0

• add multiscalar support to speed up signing and aggregating
• change errors caused by protocol violations to contain the misbehaving party
• add frost::keys::split()
• rename reconstruct_secret() to reconstruct(), make it takes a slice instead
  of a Vector, make it return SigningKey, fix it to return Error instead of an
  error string
• rename keygen_with_dealer() to generate_with_dealer()
• change SigningKey::new() to take a reference instead of a value