API Stability: External Types

[hdost]

Related to the PR #1303 if there are external types which are exposed then we can potentially break our users when an external type changes.

[cijothomas]

From SIG call:
We still need to audit all public types and ensure non_exhaustive is marked wherever appropriate.
Some of our deps are pre 1.0, we may have to 1. take that risk or 2. hide it/wrap it in our types to protect stability.