use `instance_method` to get method object #446

Job	Run time
cargo test	56s
cargo clippy	34s
make (yjit-bindgen, To fix: use patch in logs, --with-gcc=clang-12 --enable-yjit=dev)	5m 39s
make (check, RUSTC='rustc +1.58.0', 1.58.0)	9m 41s
make (check, --enable-yjit=dev)	12m 58s
make (check, --enable-yjit=dev, --yjit-call-threshold=1 --yjit-verify-ctx --yjit-code-gc, -T --yj...	17m 18s
make (test-bundled-gems, --enable-yjit=dev)	20m 14s
make (yjit-bench, --enable-yjit=dev, --yjit-stats, true)	6m 0s
YJIT Ubuntu result	1s
	1h 13m 21s

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