Rename Block -> BeaconBlock

- Update types::block
- Update ssz_helpers::SszBlock
- Update db::stores::block_store
- Add new fields to types::Block
- Update SszBlock as per new Block fields

beacon_chain/utils/ssz_helpers/src/lib.rs

@@ -4,4 +4,4 @@ extern crate types;
 extern crate ssz;
 
 pub mod attestation_ssz_splitter;
-pub mod ssz_block;
+pub mod ssz_beacon_block;