Prefer RB_OBJ_FROZEN_RAW

following the original implementation's change.
RB_TYPE_P(obj, T_OBJECT) is already checked in these places.

```
$ benchmark-driver -v --rbenv 'before --jit;after --jit' --repeat-count=12 --alternate --output=all benchmark.yml
before --jit: ruby 3.0.0dev (2020-12-19T08:27:44Z master 52b1716c78) +JIT [x86_64-linux]
after --jit: ruby 3.0.0dev (2020-12-19T08:27:44Z master 52b1716c78) +JIT [x86_64-linux]
Calculating -------------------------------------
                                 before --jit           after --jit
Optcarrot Lan_Master.nes    88.04551460097873     84.38303800957766 fps
                            88.25194345156318     85.31098251408059
                            88.34143982084871     86.60491582339496
                            88.63486879856976     88.23675694701865
                            88.85392212902701     88.23696283371444
                            89.05739427483194     88.97185459567562
                            89.08141031147311     90.16373192658857
                            89.11359420883423     90.61655686444394
                            89.80323392966130     90.77044959019291
                            90.58912189625207     90.88534596330966
                            90.59847996970350     91.34314801302897
                            90.61180456415137     93.11599164249547
```

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