Move UID handling from Base

Also clean up some of the value creation code.

graph/iterator/value_comparison_iterator.go

@@ -47,6 +47,7 @@ const (
 
 type Comparison struct {
 	Base
+	uid   uint64
 	tags  graph.Tagger
 	subIt graph.Iterator
 	op    Operator
@@ -55,15 +56,21 @@ type Comparison struct {
 }
 
 func NewComparison(sub graph.Iterator, op Operator, val interface{}, ts graph.TripleStore) *Comparison {
-	var vc Comparison
+	vc := Comparison{
+		uid:   NextUID(),
+		subIt: sub,
+		op:    op,
+		val:   val,
+		ts:    ts,
+	}
 	BaseInit(&vc.Base)
-	vc.subIt = sub
-	vc.op = op
-	vc.val = val
-	vc.ts = ts
 	return &vc
 }
 
+func (it *Comparison) UID() uint64 {
+	return it.uid
+}
+
 // Here's the non-boilerplate part of the ValueComparison iterator. Given a value
 // and our operator, determine whether or not we meet the requirement.
 func (it *Comparison) doComparison(val graph.Value) bool {