[lambda-leftovers] Underscore parameter for abstract/native methods

[Tagir Valeev]

Hello!

Currently in lambda-leftovers underscore is allowed as a parameter name for
abstract methods. This looks dubious as normally underscore means that
parameter is not used in the corresponding method/lambda/catch body, but
there's no actual body and no parameter is actually used anyways (but could
be used in abstract method implementations). I think, allowing underscores
for interface methods may encorage bad practices of defining interfaces
without readable parameter names:

interface X {
  void doSomething(int _, long _, String _);
}

Should not this be disabled?

It's even worse for native methods, because underscore assumes that
parameter is not used, but it does not limit the corresponding native
implementation from using it.

With best regards,
Tagir Valeev.