Fix aspects of the Show setup

[dwijnand]

1. Fix accidentally recursive ParamInfo instance, and test
2. Add a missing Integer instance, needed for VarianceMap, and test
3. Fix showing nested non-Showable, non-primitive values, for instance
   showing a value of type List[(Type, Int)] wasn't pretty-printing
   the Type. To fix this we need to introduce CtxShow, to defer the
   showing until we can wire a Context to it. And test. Additionally,
   we can do the non-sensical tagging with a Printer, like explainations
   does.
4. Addtionally, that ^ change brings the em/ex features to nested values
   too, and add tests for those.
5. Add some notes on how MessageLimiter works
6. Fix OrderingContraint assuming ctx.run is non null.

[som-snytt]

This should be labeled "Show-stopper".

[odersky]

The other changes look good.