std.unicode.utf8ValidateSlice: optimize implementation (#17329)

Originally inspired by Go's `utf8.Valid` function. Includes some test cases from Go's test suite.

Further optimized to be faster in all tested cases (short/long ascii/UTF8), in all release modes.

Takes advantage of SIMD for the ASCII fast path.
d68f39b541Karl Seguin committed on 10/7/2023, 3:49:21 AM· committed by GitHubparent5a4a587
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