Motivation:
codegen-kotlin generates a mix of data classes and regular classes.
For regular classes, toString() methods are also generated.
However, the output of generated toString() methods
differs from the output of default toString() methods of data classes.
Changes:
Generate toString() methods that produce the same output
as default toString() methods of data classes.
Also: rename KotlinCodegenOptions to KotlinCodeGeneratorOptions
The new name is consistent with existing names KotlinCodeGenerator and CliKotlinCodeGeneratorOptions.
Backward compatibility is ensured by turning KotlinCodegenOptions into a (deprecated) type alias.