Richard was interrogating me about decideQuantification yesterday.
I got a bit stuck on the promote_tvs part. This refactoring

 * simplifes the API of decideQuantification

 * move mkMinimalBySCs into decideQuantification (a better place for it)

 * moves promotion out of decideQuantification (where it didn't really
   fit), and comments much more fully what is going on with the promtion stuff

 * comments decideQuantification more fully

 * coments the EqPred case of quantifyPred more fully

It turned out that the theta returned by decideQuantification,
and hence by simplifyInfer, is now fully zonked, so I could remove
a zonking in TcBinds.