Code refactoring (#143)

- simplify code
- prealoc slices
- check errors in tests
- regexp Compile replaced with MustCompile
- uint* cannot be negative

pkg/core/storage/helpers.go

@@ -63,7 +63,7 @@ func HeaderHashes(s Store) ([]util.Uint256, error) {
 	})
 
 	var (
-		hashes     []util.Uint256
+		hashes     = make([]util.Uint256, 0, len(hashMap))
 		sortedKeys = make([]uint32, 0, len(hashMap))
 	)