The new `is_ascii` function is optimized to use the
`pmovmskb` vector instruction which tests the high bit in a lane.
This corresponds to the same check of whether a byte is ASCII so
ASCII validity checking can be vectorized. This instruction
does not exist on other platforms so it is likely to regress performance
and is gated to all(target_arch = "x86_64", target_feature = "sse2").
Add codegen test
Remove crate::mem import for functions included in the prelude
