vm: add OP_RETNIL for returning nil directly

Add a new opcode that returns nil without requiring LOADNIL + RETURN.
This avoids loading nil into a register by setting the return value (v)
directly. The implementation uses a separate label (L_RETURN_NIL) to
bypass v = regs[a], preserving self in regs[0] for ensure blocks.

Codegen applies peephole optimization to fuse LOADNIL + RETURN -> RETNIL.

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

src/codedump.c

@@ -399,6 +399,9 @@ codedump(mrb_state *mrb, const mrb_irep *irep, FILE *out)
     CASE(OP_RETSELF, Z):
       fprintf(out, "RETSELF\n");
       break;
+    CASE(OP_RETNIL, Z):
+      fprintf(out, "RETNIL\n");
+      break;
     CASE(OP_BREAK, B):
       fprintf(out, "BREAK\t\tR%d\t", a);
       print_lv_a(mrb, irep, a, out);