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.

library/core/src/char/methods.rs

@@ -927,7 +927,7 @@ impl char {
     #[must_use]
     #[inline]
     pub(crate) fn is_grapheme_extended(self) -> bool {
-        unicode::Grapheme_Extend(self)
+        self > '\x7f' && unicode::Grapheme_Extend(self)
     }
 
     /// Returns `true` if this `char` has one of the general categories for numbers.