Use rank 2 polymorphism for callCC

[sjshuck]

[original issue 101 by @sjshuck]

Downstream ticket in mtl: haskell/mtl#127

[sjshuck]

To summarize the discussion there that's relevant to transformers:

• there's broad consensus that this is good
• there is ambiguity about how a compiler might implement RankNTypes so it's questionable whether that should be considered one of the de facto standard extensions
• at least one person wants no RankNTypes in transformers
• I suggested we only enable higher-rank polymorphism for GHC of the appropriate minimum version
• this would be a breaking change

I notice now that MicroHs enables RankNTypes by default.

I would like to do this. I'd like a second opinion about whether this would be a breaking change. What would break?