Make variation_count an unsigned char

Since SHAPE_MAX_VARIATIONS is 8, it can easily fit inside an unsigned
char.

internal/class.h

@@ -11,6 +11,7 @@
 #include "id_table.h"           /* for struct rb_id_table */
 #include "internal/gc.h"        /* for RB_OBJ_WRITE */
 #include "internal/serial.h"    /* for rb_serial_t */
+#include "internal/static_assert.h"
 #include "ruby/internal/stdbool.h"     /* for bool */
 #include "ruby/intern.h"        /* for rb_alloc_func_t */
 #include "ruby/ruby.h"          /* for struct RBasic */
@@ -25,6 +26,7 @@ struct rb_subclass_entry {
     struct rb_subclass_entry *next;
     struct rb_subclass_entry *prev;
 };
+typedef struct rb_subclass_entry rb_subclass_entry_t;
 
 struct rb_cvar_class_tbl_entry {
     uint32_t index;
@@ -52,12 +54,15 @@ struct rb_classext_struct {
     const VALUE refined_class;
     rb_alloc_func_t allocator;
     const VALUE includer;
-    uint32_t max_iv_count;
-    uint32_t variation_count;
 #if !SHAPE_IN_BASIC_FLAGS
     shape_id_t shape_id;
 #endif
+    uint32_t max_iv_count;
+    unsigned char variation_count;
 };
+typedef struct rb_classext_struct rb_classext_t;
+
+STATIC_ASSERT(shape_max_variations, SHAPE_MAX_VARIATIONS < (1 << (sizeof(((rb_classext_t *)0)->variation_count) * CHAR_BIT)));
 
 struct RClass {
     struct RBasic basic;