Fix newtype deriving properly (un-doing Audreys patch)

Fix newtype deriving properly (un-doing Audreys patch)

The newtype-deriving mechanism generates a HsSyn case expression looking
like this
	case (d `cast` co) of { ... }
That is, the case expression scrutinises a dictionary.  This is
otherwise never seen in HsSyn, and it made the desugarer
(Check.get_unused_cons) crash in tcTyConAppTyCon.

It would really be better to generate Core in TcInstDecls (the newtype
deriving part) but I'm not going to do that today.  Instead, I made
Check.get_unused_cons a bit more robust.

Audrey tried to fix this over the weekend, but her fix was, alas, utterly
bogus, which caused mysterious failures later.  I completely undid this
change.

Anyway it should work now!