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
Some masks where defined as
```rust
const NONASCII_MASK: usize = 0x80808080_80808080u64 as usize;
```
where it was assumed that `usize` is never wider than 64, which is currently true.
To make those constants valid in a hypothetical 128-bit target, these constants have been redefined in an `usize`-width-agnostic way
```rust
const NONASCII_MASK: usize = usize::from_ne_bytes([0x80; size_of::<usize>()]);
```
There are already some cases where Rust anticipates the possibility of supporting 128-bit targets, such as not implementing `From<usize>` for `u64`.
