Ensure BitVec is initialized using a multiple of 8

I found it was panic-ing when supplied a non-power-of-zero len.

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -692,7 +692,10 @@ where
             },
         };
 
-        trace!("BeaconChain::produce_block: updating state for new block.",);
+        debug!(
+            "Produced block with {} attestations, updating state.",
+            block.body.attestations.len()
+        );
 
         per_block_processing_without_verifying_block_signature(&mut state, &block, &self.spec)?;
 

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

@@ -33,9 +33,11 @@ impl BooleanBitfield {
     }
 
     /// Create a new bitfield with the given length `initial_len` and all values set to `bit`.
-    pub fn from_elem(inital_len: usize, bit: bool) -> Self {
+    pub fn from_elem(initial_len: usize, bit: bool) -> Self {
+        // BitVec can panic if we don't set the len to be a multiple of 8.
+        let len = ((initial_len + 7) / 8) * 8;
         Self {
-            0: BitVec::from_elem(inital_len, bit),
+            0: BitVec::from_elem(len, bit),
         }
     }