stackitem: add some hint to 'seen' maps

It's empirical, we usually have one container, but four is likely to fit most
of regular cases.

pkg/vm/stackitem/item.go

@@ -1097,7 +1097,7 @@ func (i *Buffer) Len() int {
 // Values of Interop items are not deeply copied.
 // It does preserve duplicates only for non-primitive types.
 func DeepCopy(item Item) Item {
-	seen := make(map[Item]Item)
+	seen := make(map[Item]Item, typicalNumOfItems)
 	return deepCopy(item, seen)
 }
 

pkg/vm/stackitem/json.go

@@ -42,7 +42,7 @@ var ErrTooDeep = errors.New("too deep")
 //   Array, Struct -> array
 //   Map -> map with keys as UTF-8 bytes
 func ToJSON(item Item) ([]byte, error) {
-	seen := make(map[Item]sliceNoPointer)
+	seen := make(map[Item]sliceNoPointer, typicalNumOfItems)
 	return toJSON(nil, seen, item)
 }
 
@@ -260,7 +260,7 @@ func (d *decoder) decodeMap() (*Map, error) {
 
 // ToJSONWithTypes serializes any stackitem to JSON in a lossless way.
 func ToJSONWithTypes(item Item) ([]byte, error) {
-	result, err := toJSONWithTypes(item, make(map[Item]bool))
+	result, err := toJSONWithTypes(item, make(map[Item]bool, typicalNumOfItems))
 	if err != nil {
 		return nil, err
 	}

pkg/vm/stackitem/serialization.go

@@ -13,6 +13,12 @@ import (
 // (including itself).
 const MaxDeserialized = 2048
 
+// typicalNumOfItems is the number of items covering most serializaton needs.
+// It's a hint used for map creation, so it's not limiting anything, it's just
+// a microoptimization to avoid excessive reallocations. Most of the serialized
+// items are structs, so there is at least one of them.
+const typicalNumOfItems = 4
+
 // ErrRecursive is returned on attempts to serialize some recursive stack item
 // (like array including an item with reference to the same array).
 var ErrRecursive = errors.New("recursive item")
@@ -40,7 +46,7 @@ type deserContext struct {
 func Serialize(item Item) ([]byte, error) {
 	sc := serContext{
 		allowInvalid: false,
-		seen:         make(map[Item]sliceNoPointer),
+		seen:         make(map[Item]sliceNoPointer, typicalNumOfItems),
 	}
 	err := sc.serialize(item)
 	if err != nil {
@@ -69,7 +75,7 @@ func EncodeBinary(item Item, w *io.BinWriter) {
 func EncodeBinaryProtected(item Item, w *io.BinWriter) {
 	sc := serContext{
 		allowInvalid: true,
-		seen:         make(map[Item]sliceNoPointer),
+		seen:         make(map[Item]sliceNoPointer, typicalNumOfItems),
 	}
 	err := sc.serialize(item)
 	if err != nil {