Re-engineer Given flatten-skolems

The big change here is to fix an outright bug in flattening of Givens,
albeit one that is very hard to exhibit.  Suppose we have the constraint
    forall a. (a ~ F b) => ..., (forall c. ....(F b)...) ...

Then
 - we'll flatten the (F) b to a fsk, say  (F b ~ fsk1)
 - we'll rewrite the (F b) inside the inner implication to 'fsk1'
 - when we leave the outer constraint we are suppose to unflatten;
   but that fsk1 will still be there
 - if we re-simplify the entire outer implication, we'll re-flatten
   the Given (F b) to, say, (F b ~ fsk2)
Now we have two fsks standing for the same thing, and that is very
wrong.

Solution: make fsks behave more like fmvs:
 - A flatten-skolem is now a MetaTyVar, whose MetaInfo is FlatSkolTv
 - We "fill in" that meta-tyvar when leaving the implication
 - The old FlatSkol form of TcTyVarDetails is gone completely
 - We track the flatten-skolems for the current implication in
   a new field of InertSet, inert_fsks.

See Note [The flattening story] in TcFlatten.

In doing this I found various other things to fix:

* I removed the zonkSimples from TcFlatten.unflattenWanteds; it wasn't
  needed.   But I added one in TcSimplify.floatEqualities, which does
  the zonk precisely when it is needed.

* Trac #13674 showed up a case where we had
     - an insoluble Given,   e.g.  a ~ [a]
     - the same insoluble Wanted   a ~ [a]
  We don't use the Given to rewwrite the Wanted (obviously), but
  we therefore ended up reporting
      Can't deduce (a ~ [a]) from (a ~ [a])
  which is silly.

  Conclusion: when reporting errors, make the occurs check "win"
  See Note [Occurs check wins] in TcErrors