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Unshare arrays only when they become shareable #785

Job	Run time
ruby-bench (--warmup=1 --bench=1 --excludes=shipit)	12m 49s
make (check, --enable-shared --enable-load-relative)	11m 38s
make (check, i686)	30m 21s
make (check, --disable-yjit)	9m 44s
make (check, ubuntu-24.04, capi)	8m 14s
make (test-bundled-gems)	12m 38s
make (check, ubuntu-26.04-arm)	8m 40s
make (check, cppflags=-DVM_CHECK_MODE)	11m 15s
make (test-bundler-parallel, 50)	15m 42s
make (check, ubuntu-24.04, CC=clang-18 cflags=-fsanitize=address cppflags=-DUSE_MN_THREADS=0, 60)	13m 55s
make-ibm	0s
Ubuntu result	2s
	2h 14m 58s