Pin object ivars while they are being copied in a hash #8720

Job	Run time
make (check, cppflags=-DVM_CHECK_MODE)	8m 21s
make (check, i686)	8m 47s
make (check, --disable-yjit)	5m 31s
make (check, --enable-shared --enable-load-relative)	6m 1s
make (test-all TESTS=--repeat-count=2)	15m 11s
make (test-all, cppflags=-DUNIVERSAL_PARSER)	5m 10s
make (test-bundler-parallel)	19m 26s
make (test-bundled-gems)	16m 44s
Ubuntu result	1s
	1h 25m 12s

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