Fix various clippy lints

eth2/state_processing/src/block_processable.rs

@@ -194,7 +194,7 @@ fn per_block_processing_signature_optional(
             ),
             Error::BadProposerSlashing
         );
-        state.penalize_validator(proposer_slashing.proposer_index as usize, spec);
+        state.penalize_validator(proposer_slashing.proposer_index as usize, spec)?;
     }
 
     /*

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

@@ -81,6 +81,11 @@ impl BooleanBitfield {
         self.0.len()
     }
 
+    /// Returns true if `self.len() == 0`
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
     /// Returns the number of bytes required to represent this bitfield.
     pub fn num_bytes(&self) -> usize {
         self.to_bytes().len()