Use MapRange() to iterate maps

```
       │ master.bench │         fix-mapkeys.bench          │
       │    sec/op    │   sec/op     vs base               │
Map-10    3.598µ ± 1%   3.058µ ± 0%  -15.00% (p=0.002 n=6)

       │ master.bench │          fix-mapkeys.bench          │
       │     B/op     │     B/op      vs base               │
Map-10   1.844Ki ± 0%   1.328Ki ± 0%  -27.97% (p=0.002 n=6)

       │ master.bench │        fix-mapkeys.bench         │
       │  allocs/op   │ allocs/op   vs base              │
Map-10     130.0 ± 0%   128.0 ± 0%  -1.54% (p=0.002 n=6)
```

.gitignore

@@ -0,0 +1 @@
+*.test

hashstructure.go

@@ -215,8 +215,11 @@ func (w *walker) visit(v reflect.Value, opts *visitOpts) (uint64, error) {
 		// Build the hash for the map. We do this by XOR-ing all the key
 		// and value hashes. This makes it deterministic despite ordering.
 		var h uint64
-		for _, k := range v.MapKeys() {
-			v := v.MapIndex(k)
+
+		iter := v.MapRange()
+		for iter.Next() {
+			k := iter.Key()
+			v := iter.Value()
 			if includeMap != nil {
 				incl, err := includeMap.HashIncludeMap(
 					opts.StructField, k.Interface(), v.Interface())