Nicholas Nethercote 56514076ac Rework CStrUnit.
- Rename it as `MixedUnit`, because it will soon be used in more than
  just C string literals.
- Change the `Byte` variant to `HighByte` and use it only for
  `\x80`..`\xff` cases. This fixes the old inexactness where ASCII chars
  could be encoded with either `Byte` or `Char`.
- Add useful comments.
- Remove `is_ascii`, in favour of `u8::is_ascii`.
2024-01-25 12:28:11 +11:00
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2024-01-25 12:28:11 +11:00