util/deephash: avoid using sync.Pool for reflect.MapIter (#5333)

In Go 1.19, the reflect.Value.MapRange method uses "function outlining"
so that the allocation of reflect.MapIter is inlinable by the caller.
If the iterator doesn't escape the caller, it can be stack allocated.
See https://go.dev/cl/400675

Performance:

	name               old time/op    new time/op    delta
	HashMapAcyclic-24    31.9µs ± 2%    32.1µs ± 1%   ~     (p=0.075 n=10+10)

	name               old alloc/op   new alloc/op   delta
	HashMapAcyclic-24     0.00B          0.00B        ~     (all equal)

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

util/deephash/deephash.go

@@ -918,8 +918,7 @@ func (h *hasher) hashValueWithType(v addressableValue, ti *typeInfo, forceCycleC
 
 type mapHasher struct {
 	h               hasher
-	valKey, valElem valueCache      // re-usable values for map iteration
-	iter            reflect.MapIter // re-usable map iterator
+	valKey, valElem valueCache // re-usable values for map iteration
 }
 
 var mapHasherPool = &sync.Pool{
@@ -948,10 +947,6 @@ func (h *hasher) hashMap(v addressableValue, ti *typeInfo, checkCycles bool) {
 	mh := mapHasherPool.Get().(*mapHasher)
 	defer mapHasherPool.Put(mh)
 
-	iter := &mh.iter
-	iter.Reset(v.Value)
-	defer iter.Reset(reflect.Value{}) // avoid pinning v from mh.iter when we return
-
 	var sum Sum
 	if v.IsNil() {
 		sum.sum[0] = 1 // something non-zero
@@ -960,7 +955,7 @@ func (h *hasher) hashMap(v addressableValue, ti *typeInfo, checkCycles bool) {
 	k := mh.valKey.get(v.Type().Key())
 	e := mh.valElem.get(v.Type().Elem())
 	mh.h.visitStack = h.visitStack // always use the parent's visit stack to avoid cycles
-	for iter.Next() {
+	for iter := v.MapRange(); iter.Next(); {
 		k.SetIterKey(iter)
 		e.SetIterValue(iter)
 		mh.h.reset()