opaque types

[nikomatsakis]

These are not modeled at all and they are confusing AF.

[lcnr]

How to model them depends on whether we want to model their inference. If so, things get really hard. If we're fine with just taking the inferred type as an input, things get a lot easier 😊 would like to be involved here and will also writeup their impl with the new solver soon as part of the next-solver stabilization