Import apply_closure into kani_core (#3375)

This enables use of `|result|` when verifying the standard library.

library/kani_core/src/lib.rs

@@ -384,6 +384,13 @@ macro_rules! kani_intrinsics {
  #[doc(hidden)]
  #[rustc_diagnostic_item = "KaniInitContracts"]
  pub fn init_contracts() {}
+
+ /// This should only be used within contracts. The intent is to
+ /// perform type inference on a closure's argument
+ #[doc(hidden)]
+ pub fn apply_closure<T, U: Fn(&T) -> bool>(f: U, x: &T) -> bool {
+ f(x)
+ }
  }
  };
 }