Constify dsa_size_class_map and use a better type.

Author: Mark G
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/CAEeOP_Zy_FvVwcAU0UX9nkOhnoR5KN%3D0B6LWX_kv0ZuSc4wbGw%40mail.gmail.com

src/backend/utils/mmgr/dsa.c

@@ -256,7 +256,7 @@ static const uint16 dsa_size_classes[] = {
  * round the size of the object up to the next multiple of 8 bytes, and then
  * index into this array.
  */
-static char dsa_size_class_map[] = {
+static const uint8 dsa_size_class_map[] = {
 	2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 11, 11, 12, 12, 13, 13,
 	14, 14, 14, 14, 15, 15, 15, 15, 16, 16, 16, 16, 17, 17, 17, 17,
 	18, 18, 18, 18, 18, 18, 18, 18, 19, 19, 19, 19, 19, 19, 19, 19,