- Rename `unescape_raw_str_or_raw_byte_str` as
`unescape_raw_str_or_byte_str`, which is more accurate.
- Remove the unused `Mode::in_single_quotes` method.
- Make some assertions more precise, and add a missing one to
`unescape_char_or_byte`.
- Change all the assertions to `debug_assert!`, because this code is
reasonably hot, and the assertions aren't required for memory safety,
and any violations are likely to be sufficiently obvious that normal
tests will trigger them.
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