Make Debug impl optional for types

[audunska]

See #795 for motivation. If this looks like an ok approach, I'll go ahead and add more documentation.

Where is a good place to put some tests for this behavior?

[LucioFranco]

This seems like a fine approach, I would like to see a lot of tests added around the different combinations of debug impls and to also make sure the root level debug isn't a problem.

[AlJohri]

Friendly bump on this @audunska -- would you be able to add some tests? This would be a really useful feature.

[audunska]

I didn't end up prioritizing it, but I can do it now :)

[LucioFranco]

This seems like a nice addition but I think we should add some tests since there is a lot of changes to the codegen.

[audunska]

Ok, I've added some tests testing both the Message trait directly, and testing code generation. Anything else?

[LucioFranco]

Thanks for adding the tests!