clarified heap alignment, to make it potentially more stable on macOS;

--- a/src/Pure/General/bytes.ML	Wed Jun 29 22:33:08 2022 +0200
+++ b/src/Pure/General/bytes.ML	Thu Jun 30 22:49:47 2022 +0200
@@ -48,7 +48,7 @@
 
 (* primitive operations *)
 
-val chunk_size = 1024 * 1024;
+val chunk_size = 1024 * 1024 - 8;
 
 abstype T = Bytes of
   {buffer: string list, chunks: string list, m: int (*buffer size*), n: int (*chunks size*)}