Auto merge of #121138 - Swatinem:grapheme-extend-ascii, r=cuviper

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.

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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.

Author: bors