Fix clippy warnings (#1385)

## Issue Addressed

NA

## Proposed Changes

Fixes most clippy warnings and ignores the rest of them, see issue #1388.

crypto/eth2_keystore/src/json_keystore/hex_bytes.rs

@@ -14,6 +14,10 @@ impl HexBytes {
     pub fn len(&self) -> usize {
         self.0.len()
     }
+
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
 }
 
 impl From<Vec<u8>> for HexBytes {

crypto/eth2_keystore/src/json_keystore/kdf_module.rs

@@ -66,7 +66,7 @@ pub enum Prf {
 impl Prf {
     pub fn mac(&self, password: &[u8]) -> impl Mac {
         match &self {
-            _hmac_sha256 => {
+            Prf::HmacSha256 => {
                 Hmac::<Sha256>::new_varkey(password).expect("Could not derive HMAC using SHA256.")
             }
         }