Add `rb_ec_close` function to manage execution context cleanup. (#15253) #330

Job	Run time
make (check, --enable-yjit=dev, --yjit-call-threshold=1 --yjit-verify-ctx --yjit-code-gc, -T --yj...	12m 40s
make (check, --enable-yjit, --yjit)	5m 48s
cargo test	39s
YJIT macOS result	4s
	19m 11s

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