graph: make graph analysis routines safe for indeterminate iterators

This is a change in design for the graph.NodesOf family of functions. The
alternative was to provide an equivalent set of non-panicking routines in
graph for internal use. The protection that was intended with the panic
was to panic early rather than late when an indeterminate iterator exhausts
slice index space. I think in hindsight this was an error and we should
let things blow up in that (likely rare) situation.

The majority of changes are in test code. Outside the iterator package, which
is intimately tied to the determined iterator implementations, only one test
now fails if an indeterminate iterator is used, product's Modular extended
sub-graph isomorphism example, which is an algorithm that would have time
complexity issues with large iterators anyway.

graph/complement_test.go

@@ -28,7 +28,7 @@ var complementTests = []struct {
 
 func TestComplement(t *testing.T) {
 	for _, test := range complementTests {
-		n := test.g.Nodes().Len()
+		n := len(graph.NodesOf(test.g.Nodes()))
 		wantM := n * (n - 1) // Double counting edges, but no self-loops.
 
 		var gotM int
@@ -72,6 +72,10 @@ var nodeFilterIteratorTests = []struct {
 func TestNodeFilterIterator(t *testing.T) {
 	for _, test := range nodeFilterIteratorTests {
 		it := graph.NewNodeFilterIterator(test.src, test.filter, test.root)
+		if it.Len() < 0 {
+			t.Logf("don't test indeterminate iterators: %T", it)
+			continue
+		}
 		for i := 0; i < 2; i++ {
 			n := it.Len()
 			if n != test.len {