Wrap java.util.function classes nicely in Swift

We should consider wrapping the Java classes from [`java.util.function`](https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/function/package-summary.html) as part of JavaKit, either the main module or a `JavaKitFunction` module. We should probably extend those wrapper types with `callAsFunction` overloads so they can be called directly from Swift.