Add ASCII fast-path for `char::is_grapheme_extended` I discovered that `impl Debug for str` is quite slow because it ends up doing a `unicode_data::grapheme_extend::lookup` for each char, which ends up doing a binary search. This introduces a fast-path for ASCII chars which do not have this property. The `lookup` is thus completely gone from profiles. --- As a followup, maybe it’s worth implementing this fast path directly in `unicode_data` so that it can check for the lower bound directly before going to a potentially expensive binary search.