Fix warnings in Scala code
[Current master](https://github.com/kaitai-io/kaitai_struct_compiler/commit/f725faf548f42d7a7eb95f4808a6459f51284899) has 59 warnings when compiled compiler, and it is difficult for newbies in Scala to debug their changes because of this. Since I have not found any open issues about warnings, I create this