Equationally constraint the Example instance of IO to ()

[sol]

This allows for assertFailure message and throwIO err + at times it might give you slightly better type errors.

However, it's a breaking change as it breaks existing instances of the form:

Instance Example (IO MyTestResult) where
  ...

Questions:

1. How likely is this to break existing code? Has anybody seen instances of that form in the wild?
2. Are the benefits tangible enough to make this worthwhile?