unstable: fix Pattern trait implementation

I am teetering on removing this cursed implementation.

Fixes #1231

src/pattern.rs

@@ -1,4 +1,4 @@
-use core::str::pattern::{Pattern, SearchStep, Searcher};
+use core::str::pattern::{Pattern, SearchStep, Searcher, Utf8Pattern};
 
 use crate::{Matches, Regex};
 
@@ -21,6 +21,10 @@ impl<'r> Pattern for &'r Regex {
             next_match: None,
         }
     }
+
+    fn as_utf8_pattern<'p>(&'p self) -> Option<Utf8Pattern<'p>> {
+        None
+    }
 }
 
 unsafe impl<'r, 't> Searcher<'t> for RegexSearcher<'r, 't> {