Mark case objects as final

When declaring a case object, make it final.

Reason

While case objects are effectively final in Scala - you can’t extend them - they are not always flagged as such in the compiled bytecode.

Take the following code, for example:

class Test {
  case object foo
}

When foo is not flagged as final, running the class file through javap -c yields the following (truncated) output:

public class Test$foo$ implements scala.Product,scala.Serializable {
 // [...]

When it’s flagged as final, however, we get:

public final class Test$foo$ implements scala.Product,scala.Serializable {
  // [...]

To make matters worse, this is not even coherent accross Scala versions - the previous output was obtained with 2.12.7, but 2.11.8, for example, will compile both versions of Test.foo as final.

This might not seem like a big deal - as far as scalac is concerned, Test.foo is final either way and you won’t be able to extend it - not having this information in the bytecode will:

• prevent the JIT from applying any final-specific optimisations
• potentially cause Java interop issues, since javac will let you extend Test.foo.

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