- Add the required `override` modifiers to trait methods that now
  extend default methods of Java interfaces.

- Add casts in some of these methods because the Scala primitive types
  are not compatible at the type level with the (now inherited) Java
  boxed types.

- Use the boxed JFunction types in BoxingTest because the generic ones
  do not exist anymore.

- Tests in LambdaTest need to be commented out because usage from Java
  changed significantly after scala/scala#5298
  but we can still cross-build the actual library against 2.11 and
  2.12.0-M5. At some point we should consider branching.

- The various JFunction and JProcedure sources that were removed from
  Scala in scala/scala#5298 are now included
  here when building for Scala 2.12.x (except M5).

- Code that relies on specialized JFunction types being subtypes of the
  generic JFunction types will inevitably break. This is unavoidable.
  The specialized types are still part of Scala but they extend Scala
  function types directly.

- All JFunctionN types and JFunction.func methods are deprecated. They
  are provided for source compatibility between 2.11 and 2.12 only.

- The JProcedureN classes, JFunction.funcSpecialized methods and
  JFunction.proc methods are still useful in 2.12 for easy
  interoperability between Scala specialization and Java’s primitive
  types from Java code.

- The various JFunction and JProcedure sources that were removed from
  Scala in scala/scala#5298 are now included
  here when building for Scala 2.12.x (except M5).

- Code that relies on specialized JFunction types being subtypes of the
  generic JFunction types will inevitably break. This is unavoidable.
  The specialized types are still part of Scala but they extend Scala
  function types directly.

- All JFunctionN types and JFunction.func methods are deprecated. They
  are provided for source compatibility between 2.11 and 2.12 only.

- The JProcedureN classes, JFunction.funcSpecialized methods and
  JFunction.proc methods are still useful in 2.12 for easy
  interoperability between Scala specialization and Java’s primitive
  types from Java code.

- Generate JFunction and JProcedure sources:
  Just like for 2.11, we generate the sources again for 2.12 (after M5).
  I wrote a new code generator which is much simpler than the old one
  (because there is less and simpler code to generate). We can remove the
  old one when we drop support for Scala 2.11.

- Move code back from scala.runtime.java8 to scala.compat.java8:
  The standard library contains some specialized JFunction* classes in
  scala.runtime.java8, so we cannot use the same package here if we ever
  want this to work with Java 9 modules. Therefore everything is moved
  back to scala.compat.java8 where it was in 0.7.

  We have to drop compatibility with Scala 2.12.0-M5 which contains
  these classes as part of scala-library in scala.runtime.java8.

- Update to proper Scala 2.12.0-RC so we can build a release