Faster memchr, memchr2 and memchr3 in generic version #1822

Job	Run time
build-for-x86-64-but-non-sse-target	37s
miri	37m 46s
msrv	1m 8s
wasm	1m 11s
test (stable, ubuntu-latest, stable)	2m 21s
wasm-no-simd128	40s
test (beta, ubuntu-latest, beta)	1m 59s
rebar	2m 34s
test (nightly, ubuntu-latest, nightly)	1m 59s
rustfmt	8s
test (macos, macos-latest, stable)	1m 26s
test (win-msvc, windows-latest, stable)	4m 5s
test (win-gnu, windows-latest, stable-x86_64-gnu)	3m 15s
test (stable-x86, ubuntu-latest, stable, i686-unknown-linux-gnu)	2m 39s
test (stable-aarch64, ubuntu-latest, stable, aarch64-unknown-linux-gnu)	2m 40s
test (stable-powerpc64, ubuntu-latest, stable, powerpc64-unknown-linux-gnu)	2m 24s
test (stable-s390x, ubuntu-latest, stable, s390x-unknown-linux-gnu)	2m 25s
	1h 9m 17s

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