Finally make modular-frost work with alloc alone

Carries the update to `frost-schnorrkel` and `bitcoin-serai`.

crypto/frost/src/tests/mod.rs

@@ -1,4 +1,4 @@
-use std::collections::HashMap;
+use std_shims::collections::HashMap;
 
 use rand_core::{RngCore, CryptoRng};
 

crypto/frost/src/tests/nonces.rs

@@ -1,4 +1,4 @@
-use std::io::{self, Read};
+use std_shims::io::{self, Read};
 
 use zeroize::Zeroizing;
 

crypto/frost/src/tests/vectors.rs

@@ -1,8 +1,8 @@
 use core::ops::Deref;
 
-use std::collections::HashMap;
+use std_shims::collections::HashMap;
 #[cfg(test)]
-use std::str::FromStr;
+use core::str::FromStr;
 
 use zeroize::Zeroizing;