Split the stdsimd feature into separate features #1851

Job	Run time
Check Style	17s
Automatic intrinsic verification	38s
Env Override	23s
Build std_detect	1m 17s
Build Documentation	1m 38s
Test (i686-unknown-linux-gnu)	3m 20s
Test (x86_64-unknown-linux-gnu)	6m 2s
Test (x86_64-unknown-linux-gnu-emulated)	15m 52s
Test (arm-unknown-linux-gnueabihf)	3m 12s
Test (armv7-unknown-linux-gnueabihf)	16m 3s
Test (aarch64-unknown-linux-gnu)	32m 7s
Test (riscv64gc-unknown-linux-gnu)	2m 22s
Test (powerpc64le-unknown-linux-gnu)	7m 48s
Test (s390x-unknown-linux-gnu)	3m 32s
Test (wasm32-wasi)	2m 50s
Test (i586-unknown-linux-gnu)	4m 1s
Test (nvptx64-nvidia-cuda)	40s
Test (thumbv6m-none-eabi)	30s
Test (thumbv7m-none-eabi)	21s
Test (thumbv7em-none-eabi)	20s
Test (thumbv7em-none-eabihf)	16s
Test (aarch64-apple-darwin)	1m 36s
Test (x86_64-pc-windows-msvc)	9m 44s
Test (i686-pc-windows-msvc)	4m 50s
Test (x86_64-apple-darwin, macos-11)	7m 14s
	2h 6m 53s

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