Conal Eliott (Trac #1145) exposed a nasty flaw in the way in which
orphan instances are computed, when there are functional dependencies
in the class.  It took me some time to figure out what was going on,
and led to more refactoring.

Briefly:

* Elaborate comments about orphan-hood and versioning added to IfaceSyn
* The is_orph field vanishes from InstEnv.Instance
* Similarly ru_orph vanishes from CoreSyn.CoreRule
* Orphan-hood is computed in MkIface.instanceToIfaceInst, and
	MkIface.coreRuleToIfaceRule

Elsewhere just tidying up.