frost-core v2.0.0

• Updated docs
• Added missing derive(Getters) for dkg::{round1, round2}
• Added internal feature for validate_num_of_signers
• Added refresh share functionality for trusted dealer:
  frost_core::keys::refresh::{compute_refreshing_shares, refresh_share}
• Added a 'static bound to the Ciphersuite trait. This is a breaking change,
  but it's likely to not require any code changes since most ciphersuite
  implementations are probably just empty structs. The bound makes it possible
  to use frost_core::Error<C> in Box<dyn std::error::Error>.
• Added getters to round1::SecretPackage and round2::SecretPackage.
• Added a frost_core::verify_signature_share() function which allows verifying
  individual signature shares. This is not required for regular FROST usage but
  might useful in certain situations where it is desired to verify each
  individual signature share before aggregating the signature.