See if `non_exhaustive` would be useful for 1.0

[tgross35]

There are a handful of structs that users should be able to access but possibly not construct in case fields change in the future. Currently we often do this by having a single private field, but maybe #[non_exhaustive] would be better suited here.

[tgross35]

We talked about this at rustweek; we would like to use #[non_exhaustive] by default, with some very minor exceptions (e.g. iovec)