fix no-std issues and warnings

Author: conradoplg

frost-core/src/lib.rs

@@ -636,8 +636,10 @@ where
 
     Ok(signature)
 }
+
 /// Optional cheater detection feature
 /// Each share is verified to find the cheater
+#[cfg(feature = "cheater-detection")]
 fn detect_cheater<C: Ciphersuite>(
     group_commitment: &GroupCommitment<C>,
     pubkeys: &keys::PublicKeyPackage<C>,

frost-core/src/traits.rs

@@ -4,9 +4,8 @@ use core::{
     fmt::Debug,
     ops::{Add, Mul, Sub},
 };
-use std::borrow::Cow;
 
-use alloc::{collections::BTreeMap, vec::Vec};
+use alloc::{borrow::Cow, collections::BTreeMap, vec::Vec};
 use rand_core::{CryptoRng, RngCore};
 
 use crate::{