Neon fast path for str::contains

[JamieCunliffe]

Using the SIMD friendly version of the function also gives a decent speed up with Neon.

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[jhpratt]

I can't vouch for correctness.

@rustbot reroll

[Mark-Simulacrum]

decent speed up with Neon.

How do you know? Did we run benchmarks (can you provide the results)?

[JamieCunliffe]

I didn't see any benchmarks for this in tree but I did write a few quick ones, something such as:

let data = "Lorem ipsum dolor sit amet";
c.bench_function("amet", |b| b.iter(|| black_box("amet".is_contained_in(&data))));

Nightly: [41.489 ns 41.512 ns 41.534 ns]
This change: [10.610 ns 10.611 ns 10.613 ns]