Base nexting on interface satisfaction

This is done unsubtlely at the moment and there is plenty of room for
optimisation of assertion location to prevent repeated reasserting as is
done now.

graph/iterator/fixed_iterator.go

@@ -30,7 +30,6 @@ import (
 // A Fixed iterator consists of it's values, an index (where it is in the process of Next()ing) and
 // an equality function.
 type Fixed struct {
-	Base
 	uid       uint64
 	tags      graph.Tagger
 	values    []graph.Value