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/iterator/value_comparison_iterator.go

@@ -127,20 +127,15 @@ func (it *Comparison) Clone() graph.Iterator {
 	return out
 }
 
-func (it *Comparison) Next() (graph.Value, bool) {
-	var val graph.Value
-	var ok bool
-	for {
-		val, ok = graph.Next(it.subIt)
-		if !ok {
-			return nil, false
-		}
+func (it *Comparison) Next() bool {
+	for graph.Next(it.subIt) {
+		val := it.subIt.Result()
 		if it.doComparison(val) {
-			break
+			it.result = val
+			return true
 		}
 	}
-	it.result = val
-	return val, ok
+	return false
 }
 
 // DEPRECATED