Reduce allocations in sortKeysMapEncoder

Use one buffer for all values.

reflect_map.go

@@ -293,13 +293,13 @@ func (encoder *sortKeysMapEncoder) Encode(ptr unsafe.Pointer, stream *Stream) {
 	subIter := stream.cfg.BorrowIterator(nil)
 	keyValues := encodedKeyValues{}
 	for mapIter.HasNext() {
-		subStream.buf = make([]byte, 0, 64)
 		key, elem := mapIter.UnsafeNext()
+		subStreamIndex := subStream.Buffered()
 		encoder.keyEncoder.Encode(key, subStream)
 		if subStream.Error != nil && subStream.Error != io.EOF && stream.Error == nil {
 			stream.Error = subStream.Error
 		}
-		encodedKey := subStream.Buffer()
+		encodedKey := subStream.Buffer()[subStreamIndex:]
 		subIter.ResetBytes(encodedKey)
 		decodedKey := subIter.ReadString()
 		if stream.indention > 0 {
@@ -310,7 +310,7 @@ func (encoder *sortKeysMapEncoder) Encode(ptr unsafe.Pointer, stream *Stream) {
 		encoder.elemEncoder.Encode(elem, subStream)
 		keyValues = append(keyValues, encodedKV{
 			key:      decodedKey,
-			keyValue: subStream.Buffer(),
+			keyValue: subStream.Buffer()[subStreamIndex:],
 		})
 	}
 	sort.Sort(keyValues)