Optimize slice::contains for bytewise types

[SomeFlyingThing]

generalize the SliceContains implementation from u8 to all types implementing UnsignedBytewiseOrd since these types have a one byte representation compatible with the
memchr implementation

[rustbot]

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @mati865 (or someone else) some time within the next two weeks.

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