Revise implementation of overlapping type family instances.
This commit changes the syntax and story around overlapping type family instances. Before, we had "unbranched" instances and "branched" instances. Now, we have closed type families and open ones. The behavior of open families is completely unchanged. In particular, coincident overlap of open type family instances still works, despite emails to the contrary. A closed type family is declared like this: > type family F a where > F Int = Bool > F a = Char The equations are tried in order, from top to bottom, subject to certain constraints, as described in the user manual. It is not allowed to declare an instance of a closed family.