Switch to ring for hkdf (#1134)

* Switch to ring for hkdf

* Remove comments

* Make some keystore tests release only

eth2/utils/eth2_keystore/tests/tests.rs

@@ -1,4 +1,5 @@
 #![cfg(test)]
+#![cfg(not(debug_assertions))]
 
 use bls::Keypair;
 use eth2_keystore::{Error, Keystore, KeystoreBuilder};