Also on a different note, I want to use `Show` to ...

# arrow-contributors

[sam]

Also on a different note, I want to use Show to generate nicer messages when assertions fail. So users can provide better string representations of custom classes. What's your recommended way of discovery? Two options spring to mind for me - SPI or having the instances live in a pre-defined package and using Class.forName. Another problem is I don't know what to do for Show<List<String>> in either of those ideas