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/memstore/triplestore.go

@@ -132,11 +132,8 @@ func (ts *TripleStore) tripleExists(t *quad.Quad) (bool, int64) {
 	}
 	it := NewLlrbIterator(smallest_tree, "")
 
-	for {
-		val, ok := it.Next()
-		if !ok {
-			break
-		}
+	for it.Next() {
+		val := it.Result()
 		if t.Equals(&ts.triples[val.(int64)]) {
 			return true, val.(int64)
 		}