Add unicode fast path to `is_printable`
Before, it would enter the full expensive check even for normal ascii characters. Now, it skips the check for the ascii characters in `32..127`. This range was checked manually from the current behavior.
I ran the `tracing` test suite in miri, and it was really slow. I looked at a profile, and miri spent most of the time in `core::char::methods::escape_debug_ext`, where half of that was dominated by `core::unicode::printable::is_printable`. So I optimized it here.
The tracing profile:
dcbd5f5134
