Add `rb_ec_close` function to manage execution context cleanup. (#15253) #727

Job	Run time
make (check, i686)	11m 54s
make (check, --enable-shared --enable-load-relative)	7m 35s
make (test-bundler-parallel, 50)	24m 10s
make (check, cppflags=-DVM_CHECK_MODE)	8m 55s
make (check, ubuntu-24.04, capi)	10m 23s
make (check, --disable-yjit)	7m 6s
make (test-bundled-gems)	20m 18s
make-ibm	0s
Ubuntu result	2s
	1h 30m 23s

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