The case BenchmarkRange seems has some problem ?

```
// syncmap code:
b.Run(fmt.Sprintf("%T", &IntMap{}), func(b *testing.B) {
m := &IntMap{}
// setup:

// reset:
b.ResetTimer()

// perG:
perG := func(b *testing.B, pb *testing.PB, i int, m *IntMap) {
for ; pb.Next(); i++ {
m.Range(func(_, _ int) bool { return true })
}
}
var i int64
b.RunParallel(func(pb *testing.PB) {
id := int(atomic.AddInt64(&i, 1) - 1)
perG(b, pb, id*b.N, m)
})
})
```

seems the setup function is nil, then the map is empy, so at the benrchmark result, it seems vary fast