If the slot_size < RVALUE_SIZE then we would underflow in the memset.

Change example to use user-defined class instead of `Numeric`.

This struct is used for addition not only for multiplication, so
remove the word `mul`, and make the member names more descriptive.

Help the compiler know that we always get a heap object here.

These macros have been defined here and there, so collect them.

The id2ref table needs to be under a VM lock to ensure there are no race
conditions. The following script crashes:

    o = Object.new

    ObjectSpace._id2ref(o.object_id)

    10.times.map do
      Ractor.new do
        10_000.times do
          a = Object.new
          a.object_id
        end
      end
    end.map(&:value)

With:

    [BUG] Object ID seen, but not in _id2ref table: object_id=2800 object=T_OBJECT
    ruby 4.0.0dev (2025-12-06T15:15:43Z ractor-id2ref-fix e7f9abdc91) +PRISM [x86_64-linux]

    -- Control frame information -----------------------------------------------
    c:0001 p:---- s:0003 e:000002 l:y b:---- DUMMY  [FINISH]

    -- Threading information ---------------------------------------------------
    Total ractor count: 5
    Ruby thread count for this ractor: 1

    -- C level backtrace information -------------------------------------------
    miniruby(rb_print_backtrace+0x14) [0x6047d09b2dff] vm_dump.c:1105
    miniruby(rb_vm_bugreport) vm_dump.c:1450
    miniruby(rb_bug_without_die_internal+0x5f) [0x6047d066bf57] error.c:1098
    miniruby(rb_bug) error.c:1116
    miniruby(rb_gc_get_ractor_newobj_cache+0x0) [0x6047d066c8dd] gc.c:2052
    miniruby(gc_sweep_plane+0xad) [0x6047d079276d] gc/default/default.c:3513
    miniruby(gc_sweep_page) gc/default/default.c:3605
    miniruby(gc_sweep_step) gc/default/default.c:3886
    miniruby(gc_sweep+0x1ba) [0x6047d0794cfa] gc/default/default.c:4154
    miniruby(gc_start+0xbf2) [0x6047d0796742] gc/default/default.c:6519
    miniruby(heap_prepare+0xcc) [0x6047d079748c] gc/default/default.c:2090
    miniruby(heap_next_free_page) gc/default/default.c:2305
    miniruby(newobj_cache_miss) gc/default/default.c:2412
    miniruby(newobj_alloc+0xd) [0x6047d0798ff5] gc/default/default.c:2436
    miniruby(rb_gc_impl_new_obj) gc/default/default.c:2515
    miniruby(newobj_of) gc.c:996
    miniruby(rb_wb_protected_newobj_of) gc.c:1046
    miniruby(str_alloc_embed+0x28) [0x6047d08fda18] string.c:1019
    miniruby(str_enc_new) string.c:1069
    miniruby(prep_io+0x5) [0x6047d07cda14] io.c:9305
    miniruby(prep_stdio) io.c:9347
    miniruby(rb_io_prep_stdin) io.c:9365
    miniruby(thread_start_func_2+0x77c) [0x6047d093a55c] thread.c:679
    miniruby(thread_sched_lock_+0x0) [0x6047d093aacd] thread_pthread.c:2241
    miniruby(co_start) thread_pthread_mn.c:469

This reverts commit 228d13f6ed914d1e7f6bd2416e3f5be8283be865.

This commit makes default.c and mmtk.c depend on shape.h, which prevents
them from building independently.