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.

query/sexp/session.go

@@ -77,11 +77,7 @@ func (s *Session) ExecInput(input string, out chan interface{}, limit int) {
 		fmt.Println(it.DebugString(0))
 	}
 	nResults := 0
-	for {
-		_, ok := graph.Next(it)
-		if !ok {
-			break
-		}
+	for graph.Next(it) {
 		tags := make(map[string]graph.Value)
 		it.TagResults(tags)
 		out <- &tags