Merge ssz little endian branch

eth2/utils/serde_hex/src/lib.rs

@@ -38,6 +38,24 @@ impl<'de> Visitor<'de> for PrefixedHexVisitor {
     }
 }
 
+pub struct HexVisitor;
+
+impl<'de> Visitor<'de> for HexVisitor {
+    type Value = Vec<u8>;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("a hex string (irrelevant of prefix)")
+    }
+
+    fn visit_str<E>(self, value: &str) -> Result<Self::Value, E>
+    where
+        E: de::Error,
+    {
+        Ok(hex::decode(value.trim_start_matches("0x"))
+            .map_err(|e| de::Error::custom(format!("invalid hex ({:?})", e)))?)
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::*;