Ability to share data across FFI without a copy

[jfro]

We've been exploring using UniFFI for our updated version of a core library to make things easier on us than using straight C FFI code & hand written wrappers. Problem is we have some parts of the code where we'd want to minimize overhead due to low latency video use cases. So ideally when sharing something like an object (interface struct with a Vec<u8> inside for example) we'd want to be able to get the ptr/len to let the foreign swift/etc code do the final copy, avoiding the copy happening at FFI level as well.

[cadnza]

Just got here from #1485. @mhammond, can it be assumed from the comments on that issue that there aren't any plans to allow data sharing across FFI without copy?