Figure out what should happen with haskeline's exceptions dependency

As of https://github.com/judah/haskeline/commit/f61f592988c2de16c1ab10c617b8c4f6db851f3f haskeline now has a dependency on exceptions. This will be something we will need to sort out for %8.10.1. exceptions' current transitive closure would add transformers-compat, fail, and stm to our current core library set. I suspect fail and transformers-compat can be made optional. stm shouldn't pose a problem as it is already a submodule.