Fix clippy warnings (#1385)

## Issue Addressed

NA

## Proposed Changes

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

beacon_node/store/src/leveldb_store.rs

@@ -77,7 +77,7 @@ impl<E: EthSpec> KeyValueStore<E> for LevelDB<E> {
     }
 
     fn sync(&self) -> Result<(), Error> {
-        self.put_bytes_sync("sync", "sync".as_bytes(), "sync".as_bytes())
+        self.put_bytes_sync("sync", b"sync", b"sync")
     }
 
     /// Retrieve some bytes in `column` with `key`.
@@ -108,7 +108,7 @@ impl<E: EthSpec> KeyValueStore<E> for LevelDB<E> {
         self.db
             .get(self.read_options(), BytesKey::from_vec(column_key))
             .map_err(Into::into)
-            .and_then(|val| Ok(val.is_some()))
+            .map(|val| val.is_some())
     }
 
     /// Removes `key` from `column`.