make utils an internal package

internal/utils/streamframe_interval.go

@@ -0,0 +1,10 @@
+package utils
+
+import "github.com/lucas-clemente/quic-go/protocol"
+
+// ByteInterval is an interval from one ByteCount to the other
+// +gen linkedlist
+type ByteInterval struct {
+	Start protocol.ByteCount
+	End   protocol.ByteCount
+}