is_whitespace() performance improvements
This is my first rust PR, so if I miss anything obvious please let me know and I'll do my best to fix it.
This was a bit more of a challenge than I realized because, while I made working code locally and tested it against the native `is_whitespace()`, this PR required changing `src/tools/unicode-table-generator`, the code that generated the code.
I have benchmarked this locally, using criterion, and have seen meaningful performance improvements. I can add those outputs to this if you'd like, but am guessing that the perf run that `@fmease` recommended is what's needed.
I have run ` ./x.py test --stage 0 library/std` after building it locally after executing `./x.py build library`. I didn't try to build the whole compiler, but maybe I should have - any guidance would be appreciated.
If this general approach makes sense, I'll take a look at some other candidate categories, e.g., `Cc`, in the future.
Oh, and I wasn't sure whether the generated code should be included in this PR or not. I did include it.
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