Support generic containers

[vhdirk]

For the library I'm making, I also want to create bindings to other languages (like python). Mozilla's https://github.com/mozilla/uniffi-rs seems pretty promising. But...
It expects enum variants that aren't primitive types to be wrapped in an Arc. Would it make sense to have a blanket implementation of Reader and Writer for stuff like Arc, Rc and Cell?

[wcampbell0x2a]

I think that's need fine, looks like a cool project!