feat: Implement PredicateBoxExt for ?Sized

In particular, this makes `predicate::str::contains(...).boxed()` work.

CHANGELOG.md

@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 <!-- next-header -->
 ## [Unreleased] - ReleaseDate
 
+### Added
+
+- The `boxed` function is now available for predicates with an `Item` type that
+  is not `Sized`.
+
 ## [3.1.2] - 2024-07-25
 
 ## [3.1.1] - 2024-07-25

src/boxed.rs

@@ -114,4 +114,15 @@ where
     }
 }
 
-impl<P, Item> PredicateBoxExt<Item> for P where P: Predicate<Item> {}
+impl<P, Item: ?Sized> PredicateBoxExt<Item> for P where P: Predicate<Item> {}
+
+#[cfg(test)]
+mod test {
+    use crate::prelude::*;
+
+    #[test]
+    fn unsized_boxed() {
+        let p = predicate::always().boxed();
+        p.eval("4");
+    }
+}