Draft: Make Reduction abstract and use it for newtype steppers

This patch makes the interface to the Reduction datatype as abstract as possible, so that all operations are done using the combinators instead of matching on the Reduction constructor. This means that all logic pertaining to Reductions is confined to the GHC.Core.Reduction module, which gives us more flexibility in changing the implementation without affecting other parts of the compiler.

One important change is to move instNewTyCon_maybe to the GHC.Core.Reduction module, and making the normalisation functions use Reduction instead of a pair, by changing NormaliseStepResult.

This patch was split off from work I was doing on directed coercions to make things more modular, and it seemed like a good thing to have regardless.