wip: Composable OptionParam

I gave this another try after failing twice several years ago. This time I found a solution but I don't have the time to finish it now. Any takers?

- Composes for all arities
- No more need for separate OptionMapper2 and OptionMapper3
- Much nicer error messages
- Easier to use than the OptionMapper DSL

The grunt work of converting all extension method trait still needs to
be done. I haven’t examined the effect on compiler performance, either.