load.c: consolidate irep allocation for .mrb loading

Pack pool/syms/reps arrays into a single calloc with the irep struct,
reducing 4 allocations per irep to 1. Arrays are ordered by descending
alignment (pool/reps/syms) to eliminate inter-array padding.

Co-authored-by: Claude <noreply@anthropic.com>

include/mruby/irep.h

@@ -82,6 +82,7 @@ struct mrb_irep {
 #define MRB_ISEQ_NO_FREE 1
 #define MRB_IREP_NO_FREE 2
 #define MRB_IREP_STATIC (MRB_ISEQ_NO_FREE | MRB_IREP_NO_FREE)
+#define MRB_IREP_CONSOLIDATED 4  /* pool/syms/reps packed with irep struct */
 
 MRB_API mrb_irep *mrb_add_irep(mrb_state *mrb);