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Add `opt_in` crate feature #247

Job	Run time
Cross (sparcv9-sun-solaris)	33s
Tier 2 (x86_64-unknown-fuchsia)	14s
Apple Other	47s
Cross (x86_64-unknown-illumos)	24s
Tier 2 (x86_64-unknown-redox)	15s
Tier 2 (x86_64-fortanix-unknown-sgx)	15s
Linux x32 (x86_64-unknown-linux-gnux32)	26s
Tier 3 (aarch64-kmc-solid_asp3)	25s
Web no_std	27s
Tier 3 (aarch64-unknown-nto-qnx710)	24s
No Atomics	12s
Web no_std with Atomics	31s
Tier 3 (armv6k-nintendo-3ds)	19s
Web	30s
Tier 3 (armv7-sony-vita-newlibeabihf)	19s
Web with Atomics	32s
Tier 3 (i686-unknown-hurd-gnu)	25s
RDRAND UEFI (x86_64-unknown-uefi)	29s
Tier 3 (riscv32imc-esp-espidf)	22s
RDRAND UEFI (i686-unknown-uefi)	28s
Tier 3 (x86_64-unknown-hermit)	21s
WasmV1 no_std	22s
Tier 3 (x86_64-wrs-vxworks)	19s
WasmV1 no_std with Atomics	25s
Tier 3 (x86_64-unknown-dragonfly)	21s
RNDR	22s
Tier 3 (x86_64-unknown-haiku)	21s
	10m 48s

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