Add @Covariant and @Contravariant annotations

As a library developer, I want to be able to add some annotations to mark the variance of the type parameters of a class. IDE can give the following hint based on this annotation:
- Report a warning when contravariant type parameter T is used as a function parameter type
- Report a warning when covariant type parameter T is used as a function return type
- For function parameter types, prompt user to use `Function` (take `java.util.function.Function` as an example) instead of `Function`

And the jvm-based language can choose to understand this annotation (e.g. kotlin can understand `class C<@Covariant T>` as `class C`. This is convenient for developers to provide Kotlin-friendly APIs using pure Java.