Fix clippy lints

eth2/utils/boolean-bitfield/src/lib.rs

@@ -226,7 +226,7 @@ impl Decodable for BooleanBitfield {
 // as the BitVec library and the hex-parser use opposing bit orders.
 fn reverse_bit_order(mut bytes: Vec<u8>) -> Vec<u8> {
     bytes.reverse();
-    bytes.into_iter().map(|b| b.swap_bits()).collect()
+    bytes.into_iter().map(LookupReverse::swap_bits).collect()
 }
 
 impl Serialize for BooleanBitfield {

eth2/utils/fixed_len_vec/src/lib.rs

@@ -22,6 +22,10 @@ impl<T, N: Unsigned> FixedLenVec<T, N> {
         self.vec.len()
     }
 
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
     pub fn capacity() -> usize {
         N::to_usize()
     }

eth2/utils/hashing/src/lib.rs

@@ -35,7 +35,7 @@ pub fn merkle_root(values: &[Vec<u8>]) -> Option<Vec<u8>> {
     }
 
     // the root hash will be at index 1
-    return Some(o[1].clone());
+    Some(o[1].clone())
 }
 
 #[cfg(test)]