Let Filter extend Predicate<Context>, Mapper extend Function<Context, E>, Content extend Function<Context, String>, Each extend Consumer<Context>

[lukaseder]

Our Filter type is just a JDK Predicate, which was offered with Java 6 support in mind. Now that Java 6 support is being dropped, we can let Filter extend Predicate, and replace all references to Filter by Predicate<? super Context> throughout the API.