Java2Swift: Consider using implicitly unwrapped optionals for field types and method result types

Java's references are implicitly nullable. Right now, Java2Swift brings them all in as optional types... but that means we need to deal with the optionality everywhere in Swift. We should consider using implicitly-unwrapped optionals for field types and method result types, rather than regular optionals, like Swift does when interoperating with C code that has unannotated pointers.