Simplify Nexter interface

This change allows a Nexter to be used in the same manner as a scanner
using a for graph.Next(it) {} construction.

It is important that graph.Next(it) and any associated it.Result() calls
operate on the same iterator.

graph/mongo/triplestore_iterator_optimize.go

@@ -37,10 +37,10 @@ func (ts *TripleStore) optimizeLinksTo(it *iterator.LinksTo) (graph.Iterator, bo
 	if primary.Type() == graph.Fixed {
 		size, _ := primary.Size()
 		if size == 1 {
-			val, ok := graph.Next(primary)
-			if !ok {
-				panic("Sizes lie")
+			if !graph.Next(primary) {
+				panic("unexpected size during optimize")
 			}
+			val := primary.Result()
 			newIt := ts.TripleIterator(it.Direction(), val)
 			nt := newIt.Tagger()
 			nt.CopyFrom(it)