[java] Respect interface methods with Code attributes

[Simn]

This consists of these steps:

• If while loading a jar we come across an interface method that has a Code attribute, add the @:java.default metadata to the field.
• If while typing a field we come across the @:java.default metadata, add the CfDefault flag to the class field.
• While checking interface implementations, if there's a missing interface method that has the CfDefault flag, create a copy of it on the implementing class the same way we would for abstract classes implementing interfaces. This step is necessary to satisfy the compiler's field resolution semantics.

Interestingly, I had to add a check to both the Java and the JVM generators to ignore methods with the CfExtern flag. That's probably because our post-processing doesn't touch interface fields.

TODO:

• Figure out if we have to do something special when interfaces extend each other. Also check the situation where a non-extern interface extends an extern interface with a default method.
• Write more tests I guess.