Vectorized SliceContains

[LaihoE]

Godbolt for the u32 case: https://rust.godbolt.org/z/exT9xYWGs

Unsure about:

• Should align_to be used? It didn't seem to matter in my benchmark but maybe I was lucky with alignment?
• Should u8/i8 also be implemented? Currently uses memchr (SWAR)

Some benchmarks on x86 (contains called on an array with no matches, worst case may be slightly worse):

Large N

Small N

[rustbot]

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[workingjubilee]

Should align_to be used? It didn't seem to matter in my benchmark but maybe I was lucky with alignment?

Probably not. It will probably not be beneficial on x86 or aarch64 systems.