Introduce "object shapes" (hidden classes) that share IV key
layouts across objects with the same instance variable assignment
order. This eliminates per-object key storage overhead.
Memory savings: ~22% heap reduction for object-heavy workloads
(e.g., 150k objects with 2-6 IVs). Per-object: 40->24 bytes
for 2 IVs. Objects exceeding 16 IVs or using
remove_instance_variable fall back to traditional iv_tbl.
Co-authored-by: Claude <noreply@anthropic.com>
Yukihiro "Matz" Matsumoto
8d10056aff