Remove Bitfield (#28)

* Remove bitfield
* Use new release of ethereum_ssz

Author: macladson

Cargo.toml

@@ -13,7 +13,7 @@ categories = ["cryptography::cryptocurrencies"]
 [dependencies]
 tree_hash = "0.8.0"
 ethereum_serde_utils = "0.7.0"
-ethereum_ssz = "0.7.0"
+ethereum_ssz = "0.8.0"
 serde = "1.0.0"
 serde_derive = "1.0.0"
 typenum = "1.12.0"