Add option to run each example in its own ghci session

[martijnbastiaan]

While we already run each module in its own ghci session, we could go further and run each example in its own session. This would ensure tests run in isolation. I decided not to pursue this for the first release due to the massive performance hit testing would take. Still, the performance hit might be worth it for some users.

Note: we already run :reload before running a test in a module. This only resets binders, but not plugin loads, GHC extension settings, ..