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Add `opt_in` crate feature #245

Job	Run time
Cross (sparcv9-sun-solaris)	35s
Linux x32 (x86_64-unknown-linux-gnux32)	29s
Cross (x86_64-unknown-illumos)	34s
Apple Other	1m 14s
No Atomics	21s
RDRAND UEFI (x86_64-unknown-uefi)	33s
RNDR	20s
RDRAND UEFI (i686-unknown-uefi)	35s
Tier 2 (x86_64-unknown-fuchsia)	19s
Tier 2 (x86_64-unknown-redox)	15s
Tier 2 (x86_64-fortanix-unknown-sgx)	15s
Tier 3 (aarch64-kmc-solid_asp3)	24s
Tier 3 (aarch64-unknown-nto-qnx710)	24s
Web no_std	24s
Tier 3 (armv6k-nintendo-3ds)	23s
Web no_std with Atomics	23s
Tier 3 (armv7-sony-vita-newlibeabihf)	21s
Web	29s
Tier 3 (i686-unknown-hurd-gnu)	21s
Web with Atomics	29s
Tier 3 (riscv32imc-esp-espidf)	19s
WasmV1 no_std	21s
Tier 3 (x86_64-unknown-hermit)	22s
Tier 3 (x86_64-wrs-vxworks)	21s
WasmV1 no_std with Atomics	24s
Tier 3 (x86_64-unknown-dragonfly)	22s
Tier 3 (x86_64-unknown-haiku)	24s
	11m 41s

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