Fix clippy warnings (#1385)

## Issue Addressed

NA

## Proposed Changes

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

crypto/eth2_key_derivation/src/derived_key.rs

@@ -81,7 +81,7 @@ fn derive_child_sk(parent_sk: &[u8], index: u32) -> SecretHash {
 ///
 /// Equivalent to `HKDF_mod_r` in EIP-2333.
 fn hkdf_mod_r(ikm: &[u8]) -> SecretHash {
-    let prk = hkdf_extract("BLS-SIG-KEYGEN-SALT-".as_bytes(), ikm);
+    let prk = hkdf_extract(b"BLS-SIG-KEYGEN-SALT-", ikm);
     let okm = &hkdf_expand(prk, MOD_R_L);
     mod_r(okm.as_bytes())
 }

crypto/eth2_key_derivation/src/plain_text.rs

@@ -16,6 +16,11 @@ impl PlainText {
         self.0.len()
     }
 
+    /// Checks whether `self` is empty
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
     /// Returns a reference to the underlying bytes.
     pub fn as_bytes(&self) -> &[u8] {
         &self.0