Replace rand with rand_core where possible

Turns out rand_core offers OsRng.

coins/monero/tests/rpc.rs

@@ -1,4 +1,4 @@
-use rand::rngs::OsRng;
+use rand_core::OsRng;
 
 use curve25519_dalek::constants::ED25519_BASEPOINT_TABLE;
 

coins/monero/tests/send.rs

@@ -4,7 +4,7 @@ use std::collections::HashMap;
 
 use lazy_static::lazy_static;
 
-use rand::rngs::OsRng;
+use rand_core::OsRng;
 
 #[cfg(feature = "multisig")]
 use blake2::{digest::Update, Digest, Blake2b512};