This allows us to swap in rb_id_table_memsize for st_memsize
(which takes a "const st_table *") more easily.

It also makes sense to do the same for rb_id_table_size,
too; as the table cannot be altered when accessing size.

* id_table.h (rb_id_table_size): const arg
  (rb_id_table_memsize): ditto
* id_table.c (st_id_table_size): ditto
  (st_id_table_memsize): ditto
  (list_id_table_size): ditto
  (list_id_table_memsize): ditto
  (hash_id_table_size): ditto
  (hash_id_table_memsize): ditto
  (mix_id_table_size): ditto
  (mix_id_table_memsize): ditto

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52428 b2dd03c8-39d4-4d8f-98ff-823fe69b080e