simd: reapply and fix split cursor advancing #249

Job	Run time
msrv (x64)	2m 3s
msrv (aarch64)	2m 30s
SIMD +sse4.2 on stable	20s
check x86	25s
SIMD +sse4.2 on stable	20s
Test aarch64 (neon)	41s
SIMD +avx2 on stable	22s
Test stable on ubuntu-latest	24s
SIMD +avx2 on stable	28s
clippy_check	21s
Test stable on windows-latest	2m 17s
SIMD +sse4.2,+avx2 on stable	26s
Test with Miri	1m 21s
Test stable on macOS-latest	23s
SIMD +sse4.2,+avx2 on stable	21s
SIMD +sse4.2 on beta	30s
Test beta on ubuntu-latest	30s
SIMD +sse4.2 on beta	27s
Test beta on windows-latest	1m 17s
SIMD +avx2 on beta	29s
Test beta on macOS-latest	28s
SIMD +avx2 on beta	26s
SIMD +sse4.2,+avx2 on beta	27s
Test nightly on ubuntu-latest	29s
SIMD +sse4.2,+avx2 on beta	29s
Test nightly on windows-latest	1m 55s
SIMD +sse4.2 on nightly	27s
Test nightly on macOS-latest	35s
SIMD +sse4.2 on nightly	25s
SIMD +avx2 on nightly	26s
SIMD +avx2 on nightly	28s
SIMD +sse4.2,+avx2 on nightly	26s
SIMD +sse4.2,+avx2 on nightly	26s
CI is green	1s
	23m 23s

Provide feedback