Fix #642 - Allow rows in instance contexts

Original discussion here: https://github.com/purescript/purescript/issues/642
As for unifying TUnknowns with everything, it breaks these tests:
```
Assert C:\purescript\purescript\examples\failing\UnknownValue.purs does not compile
Assert C:\purescript\purescript\examples\failing\UnknownType.purs does not compile
Assert C:\purescript\purescript\examples\failing\UnifyInTypeInstanceLookup.purs does not compile
Should not have compiled
```
But it seems it's ok to unify them with just skolems (and it fixes the problem).