Cleanup shreds to remove FirstShred data structure (#5789)

* Cleanup shreds to remove FirstShred data structure

* Also reduce size used by parent slot information in shred header

* clippy

* fixes

* fix chacha test

core/src/chacha.rs

@@ -153,7 +153,7 @@ mod tests {
         hasher.hash(&buf[..size]);
 
         //  golden needs to be updated if blob stuff changes....
-        let golden: Hash = "EdYYuAuDPVY7DLNeCtPWAKipicx2KjsxqD2PZ7oxVmHE"
+        let golden: Hash = "HVUt3uy4it4NgbGgYiG9C3gnvxDCzkgqwzzSCM9N4sSt"
             .parse()
             .unwrap();