shape.h: Make attr_index_t uint8_t

Given `SHAPE_MAX_NUM_IVS 80`, we transition to TOO_COMPLEX
way before we could overflow a 8bit counter.

This reduce the size of `rb_shape_t` from 32B to 24B.

If we decide to raise `SHAPE_MAX_NUM_IVS` we can always increase
that type again.

shape.c

@@ -865,6 +865,7 @@ Init_default_shapes(void)
     hash_fallback_shape->size_pool_index = 0;
     RUBY_ASSERT(OBJ_TOO_COMPLEX_SHAPE_ID == (GET_SHAPE_TREE()->next_shape_id - 1));
     RUBY_ASSERT(rb_shape_id(hash_fallback_shape) == OBJ_TOO_COMPLEX_SHAPE_ID);
+    fprintf(stderr, "sizeof(rb_shape_t) => %lu\n", sizeof(rb_shape_t));
 }
 
 void

shape.h

@@ -6,12 +6,11 @@
 #if (SIZEOF_UINT64_T <= SIZEOF_VALUE)
 #define SIZEOF_SHAPE_T 4
 #define SHAPE_IN_BASIC_FLAGS 1
-typedef uint32_t attr_index_t;
 #else
 #define SIZEOF_SHAPE_T 2
 #define SHAPE_IN_BASIC_FLAGS 0
-typedef uint16_t attr_index_t;
 #endif
+typedef uint8_t attr_index_t;
 
 #define MAX_IVARS (attr_index_t)(-1)
 
@@ -44,7 +43,7 @@ struct rb_shape {
     struct rb_id_table * edges; // id_table from ID (ivar) to next shape
     ID edge_name; // ID (ivar) for transition from parent to rb_shape
     attr_index_t next_iv_index;
-    uint32_t capacity; // Total capacity of the object with this shape
+    attr_index_t capacity; // Total capacity of the object with this shape
     uint8_t type;
     uint8_t size_pool_index;
     shape_id_t parent_id;