Pin object ivars while they are being copied in a hash #8718

Job	Run time
make (check, cppflags=-DVM_CHECK_MODE)	7m 17s
make (check, i686)	8m 37s
make (check, --disable-yjit)	5m 52s
make (check, --enable-shared --enable-load-relative)	6m 29s
make (test-all TESTS=--repeat-count=2)	15m 13s
make (test-all, cppflags=-DUNIVERSAL_PARSER)	5m 20s
make (test-bundler-parallel)	20m 6s
make (test-bundled-gems)	15m 59s
Ubuntu result	1s
	1h 24m 54s

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