Import apply_closure into kani_core (model-checking#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)
+            }
         }
     };
 }