- Implement comment nesting (the implementation is quite ugly at present
and is not quite correct; note the big comment in that area).
- Highlight invalid escape sequences as errors.
- Fix up various inconsistencies and incorrectnesses in number
highlighting.
- Update prelude items (``std::io::{Buffer, Writer, Reader, Seek}``).
- Highlight the ``proc`` keyword.
- Remove %-formatting sequence highlighting (a relic of old formatting).
- Don't highlight TODO in strings (it's unconventional).
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