move readUint64 to new file utils.go

utils.go

@@ -0,0 +1,15 @@
+package quic
+
+import "io"
+
+func readUint64(b io.ByteReader, length uint8) (uint64, error) {
+	var res uint64
+	for i := uint8(0); i < length; i++ {
+		bt, err := b.ReadByte()
+		if err != nil {
+			return 0, err
+		}
+		res = res<<8 + uint64(bt)
+	}
+	return res, nil
+}