Move UID handling from Base

Also clean up some of the value creation code.

graph/iterator/fixed_iterator.go

@@ -31,6 +31,7 @@ import (
 // an equality function.
 type Fixed struct {
 	Base
+	uid       uint64
 	tags      graph.Tagger
 	values    []graph.Value
 	lastIndex int
@@ -55,14 +56,19 @@ func newFixed() *Fixed {
 
 // Creates a new Fixed iterator with a custom comparitor.
 func NewFixedIteratorWithCompare(compareFn Equality) *Fixed {
-	var it Fixed
+	it := Fixed{
+		uid:    NextUID(),
+		values: make([]graph.Value, 0, 20),
+		cmp:    compareFn,
+	}
 	BaseInit(&it.Base)
-	it.values = make([]graph.Value, 0, 20)
-	it.lastIndex = 0
-	it.cmp = compareFn
 	return &it
 }
 
+func (it *Fixed) UID() uint64 {
+	return it.uid
+}
+
 func (it *Fixed) Reset() {
 	it.lastIndex = 0
 }