Add `SliceExt` #354

Job	Run time
careful	1m 2s
clippy	19s
rustfmt	7s
build (stable, wasm32-unknown-unknown)	41s
miri (s390x-unknown-linux-gnu)	58s
doc	21s
build (stable, thumbv7em-none-eabi)	55s
test (stable)	28s
test (1.85.0)	44s
miri (x86_64-unknown-linux-gnu)	45s
build (stable, armv7a-none-eabi)	48s
build (1.85.0, thumbv7em-none-eabi)	52s
build (1.85.0, wasm32-unknown-unknown)	51s
build (1.85.0, armv7a-none-eabi)	49s
	9m 40s

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