Remove default hasher (#4739)

* remove default hasher from decl_storage!

* fix decl_storage declarations

substrate/frame/babe/src/lib.rs

@@ -145,7 +145,7 @@ decl_storage! {
 		/// We reset all segments and return to `0` at the beginning of every
 		/// epoch.
 		SegmentIndex build(|_| 0): u32;
-		UnderConstruction: map u32 => Vec<[u8; 32 /* VRF_OUTPUT_LENGTH */]>;
+		UnderConstruction: map hasher(blake2_256) u32 => Vec<[u8; 32 /* VRF_OUTPUT_LENGTH */]>;
 
 		/// Temporary value (cleared at block finalization) which is `Some`
 		/// if per-block initialization has already been called for current block.