Move iterators into separate package

Also reduce API exposure and use standard library more - and fix bugs I
previously introduces in mongo.

graph/iterator/mock_ts_test.go

@@ -0,0 +1,60 @@
+// Copyright 2014 The Cayley Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package iterator
+
+// A quickly mocked version of the TripleStore interface, for use in tests.
+// Can better used Mock.Called but will fill in as needed.
+
+import (
+	"github.com/stretchrcom/testify/mock"
+
+	"github.com/google/cayley/graph"
+)
+
+type TestTripleStore struct {
+	mock.Mock
+}
+
+func (ts *TestTripleStore) GetIdFor(s string) graph.TSVal {
+	args := ts.Mock.Called(s)
+	return args.Get(0)
+}
+func (ts *TestTripleStore) AddTriple(*graph.Triple)             {}
+func (ts *TestTripleStore) AddTripleSet([]*graph.Triple)        {}
+func (ts *TestTripleStore) GetTriple(graph.TSVal) *graph.Triple { return &graph.Triple{} }
+func (ts *TestTripleStore) GetTripleIterator(d graph.Direction, i graph.TSVal) graph.Iterator {
+	args := ts.Mock.Called(d, i)
+	return args.Get(0).(graph.Iterator)
+}
+func (ts *TestTripleStore) GetNodesAllIterator() graph.Iterator   { return &Null{} }
+func (ts *TestTripleStore) GetTriplesAllIterator() graph.Iterator { return &Null{} }
+func (ts *TestTripleStore) GetIteratorByString(string, string, string) graph.Iterator {
+	return &Null{}
+}
+func (ts *TestTripleStore) GetNameFor(v graph.TSVal) string {
+	args := ts.Mock.Called(v)
+	return args.Get(0).(string)
+}
+func (ts *TestTripleStore) Size() int64 { return 0 }
+func (ts *TestTripleStore) DebugPrint() {}
+func (ts *TestTripleStore) OptimizeIterator(it graph.Iterator) (graph.Iterator, bool) {
+	return &Null{}, false
+}
+func (ts *TestTripleStore) FixedIterator() graph.FixedIterator {
+	return NewFixedIteratorWithCompare(BasicEquality)
+}
+func (ts *TestTripleStore) Close()                                                      {}
+func (ts *TestTripleStore) GetTripleDirection(graph.TSVal, graph.Direction) graph.TSVal { return 0 }
+func (ts *TestTripleStore) RemoveTriple(t *graph.Triple)                                {}