Issue erlang#8099: Convince compiler to use the new 'private append' …

…optimisation and ensure we start appending from an empty binary

lib/stdlib/src/binary.erl

@@ -951,7 +951,8 @@ _Example:_
 join([], _Separator) -> <<>>;
 join([H], _Separator) -> H;
 join([H | T], Separator) ->
-    join(T, Separator, H).
+    Acc = <<>>,
+    join(T, Separator, <<Acc/binary, H/binary>>).
 
 -spec join([binary()], binary(), binary()) -> binary().
 join([], _Separator, Acc) -> Acc;